diff --git a/.agents/skills/excalidraw-diagram b/.agents/skills/excalidraw-diagram
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+../../.claude/skills/excalidraw-diagram
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diff --git a/.claude/skills/excalidraw-diagram/LICENSE.txt b/.claude/skills/excalidraw-diagram/LICENSE.txt
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+This skill is adapted from the "excalidraw-diagram-generator" skill in
+https://github.com/github/awesome-copilot (skills/excalidraw-diagram-generator).
+
+Files derived from upstream:
+  references/excalidraw-schema.md   (unchanged, openmlsys note added at top)
+  references/element-types.md       (unchanged, openmlsys note added at top)
+  scripts/add-arrow.py              (label font size 14 -> 16, note in docstring)
+  templates/*.excalidraw            (fontFamily -> 5, bound text labels fixed,
+                                     font size >= 16)
+New for openmlsys: SKILL.md (rewritten), references/openmlsys-conventions.md,
+scripts/check_figures.py.
+
+The upstream license follows.
+
+MIT License
+
+Copyright GitHub, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/.claude/skills/excalidraw-diagram/SKILL.md b/.claude/skills/excalidraw-diagram/SKILL.md
new file mode 100644
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+---
+name: excalidraw-diagram
+description: Draw figures for the openmlsys "Machine Learning Systems" book with Excalidraw. Use when a chapter needs a new or redrawn diagram (architecture, pipeline, flowchart, data flow, sequence, mind map), or when asked to "draw", "diagram", "make a figure" or "create an Excalidraw file" for this repo. Produces an editable .excalidraw source plus an exported SVG/PNG under v1/img/chXX/, with EN/ZH variants when the figure contains text.
+---
+
+# Excalidraw figures for openmlsys
+
+Every figure in the book has two files side by side: the editable `.excalidraw`
+source and the exported `.svg` (preferred) or `.png` that the Markdown
+references. Read [references/openmlsys-conventions.md](references/openmlsys-conventions.md)
+before drawing. It covers paths, house style, dark mode and bilingual variants.
+
+## Workflow
+
+1. **Plan the figure.** Pick the diagram type and list the nodes, edges and
+   labels. Keep it to about 20 elements. If it needs more, split it into
+   several figures.
+2. **Choose the paths.** Images live in `v1/img/ch<NN>/`, which every chapter
+   tree reaches through its `img` symlink. Name files in lowercase with
+   hyphens, e.g. `v1/img/ch03/tensor-core-dataflow.excalidraw`. v1 and v2
+   share this folder, so check that the name is not taken.
+3. **Write the scene.** Start from a file in [templates/](templates/) or write
+   the JSON yourself. The schema is in
+   [references/excalidraw-schema.md](references/excalidraw-schema.md) and the
+   element specs are in [references/element-types.md](references/element-types.md).
+   House rules:
+   - Text uses `fontFamily: 5` (Excalifont), or `3` for code. Use `fontSize`
+     16 or more and the dark stroke `#1e1e1e`.
+   - A label inside a shape is a **separate `text` element** with
+     `containerId` set to the shape. The shape must list it in
+     `boundElements`. A `text` property on a rectangle is silently ignored.
+   - Use pastel fills, dark strokes and no white or light-colored text (see
+     the palette in the conventions file).
+   - To add a connector to an existing file, run
+     `python3 .claude/skills/excalidraw-diagram/scripts/add-arrow.py <file> x1 y1 x2 y2 [--label T] [--style dashed]`.
+4. **Make the ZH variant** if the figure contains words. Copy the scene to
+   `<name>-zh.excalidraw` and translate only the text. Then widen any boxes
+   that the Chinese labels overflow. Figures without text use one shared file.
+5. **Lint.** Run `python3 .claude/skills/excalidraw-diagram/scripts/check_figures.py <files or dirs>`.
+   It needs only the standard library. Fix every ERROR.
+6. **Export** each scene to `<name>.svg` (or `<name>.png` at 2x) next to its
+   source: background off, dark mode off, light theme. You can export with the
+   Excalidraw MCP, excalidraw.com or the VS Code Excalidraw extension (steps in
+   the conventions file). If you have no tool that can render, stop here. Tell
+   the contributor which files still need exporting. Do not hand-write an SVG.
+   **Never export or reference a PDF.** mdBook pages cannot display PDF images.
+7. **Reference** the image from the chapter with a relative path:
+
+   ```markdown
+   ![Data flow of a training step](../img/ch06/training-step-dataflow.svg)
+   :width:`700px`
+   :label:`fig-training-step-dataflow`
+   ```
+
+   Use the `-zh.svg` variant in `v2/zh_chapters/...`. Re-run `check_figures.py`
+   on the edited `.md` files to confirm that the paths resolve.
+
+## Deliverables checklist
+
+- [ ] `<name>.excalidraw` + exported `<name>.svg|png` (and `-zh` pair if the figure has text)
+- [ ] `check_figures.py` reports 0 errors for the scenes and the edited chapter files
+- [ ] Markdown references the SVG/PNG, never `.pdf` or `.excalidraw`
+- [ ] Commit the source and the export together
+
+## Attribution
+
+Adapted from the `excalidraw-diagram-generator` skill in
+[github/awesome-copilot](https://github.com/github/awesome-copilot/tree/main/skills/excalidraw-diagram-generator)
+(MIT, see [LICENSE.txt](LICENSE.txt)).
diff --git a/.claude/skills/excalidraw-diagram/references/element-types.md b/.claude/skills/excalidraw-diagram/references/element-types.md
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+# Excalidraw Element Types Guide
+
+> **openmlsys note:** This file comes from github/awesome-copilot (MIT, see `../LICENSE.txt`) and is kept unchanged. Where it shows `fontFamily` 1 to 3, use `5` (Excalifont, or `3` for code) instead. House style and labels inside shapes are covered in [openmlsys-conventions.md](openmlsys-conventions.md).
+
+Detailed specifications for each Excalidraw element type with visual examples and use cases.
+
+## Element Type Overview
+
+| Type | Visual | Primary Use | Text Support |
+|------|--------|-------------|--------------|
+| `rectangle` | □ | Boxes, containers, process steps | ✅ Yes |
+| `ellipse` | ○ | Emphasis, terminals, states | ✅ Yes |
+| `diamond` | ◇ | Decision points, choices | ✅ Yes |
+| `arrow` | → | Directional flow, relationships | ❌ No (use separate text) |
+| `line` | — | Connections, dividers | ❌ No |
+| `text` | A | Labels, annotations, titles | ✅ (Its purpose) |
+
+---
+
+## Rectangle
+
+**Best for:** Process steps, entities, data stores, components
+
+### Properties
+
+```typescript
+{
+  type: "rectangle",
+  roundness: { type: 3 },  // Rounded corners
+  text: "Step Name",       // Optional embedded text
+  fontSize: 20,
+  textAlign: "center",
+  verticalAlign: "middle"
+}
+```
+
+### Use Cases
+
+| Scenario | Configuration |
+|----------|---------------|
+| **Process step** | Green background (`#b2f2bb`), centered text |
+| **Entity/Object** | Blue background (`#a5d8ff`), medium size |
+| **System component** | Light color, descriptive text |
+| **Data store** | Gray/white, database-like label |
+
+### Size Guidelines
+
+| Content | Width | Height |
+|---------|-------|--------|
+| Single word | 120-150px | 60-80px |
+| Short phrase (2-4 words) | 180-220px | 80-100px |
+| Sentence | 250-300px | 100-120px |
+
+### Example
+
+```json
+{
+  "type": "rectangle",
+  "x": 100,
+  "y": 100,
+  "width": 200,
+  "height": 80,
+  "backgroundColor": "#b2f2bb",
+  "text": "Validate Input",
+  "fontSize": 20,
+  "textAlign": "center",
+  "verticalAlign": "middle",
+  "roundness": { "type": 3 }
+}
+```
+
+---
+
+## Ellipse
+
+**Best for:** Start/end points, states, emphasis circles
+
+### Properties
+
+```typescript
+{
+  type: "ellipse",
+  text: "Start",
+  fontSize: 18,
+  textAlign: "center",
+  verticalAlign: "middle"
+}
+```
+
+### Use Cases
+
+| Scenario | Configuration |
+|----------|---------------|
+| **Flow start** | Light green, "Start" text |
+| **Flow end** | Light red, "End" text |
+| **State** | Soft color, state name |
+| **Highlight** | Bright color, emphasis text |
+
+### Size Guidelines
+
+For circular shapes, use `width === height`:
+
+| Content | Diameter |
+|---------|----------|
+| Icon/Symbol | 60-80px |
+| Short text | 100-120px |
+| Longer text | 150-180px |
+
+### Example
+
+```json
+{
+  "type": "ellipse",
+  "x": 100,
+  "y": 100,
+  "width": 120,
+  "height": 120,
+  "backgroundColor": "#d0f0c0",
+  "text": "Start",
+  "fontSize": 18,
+  "textAlign": "center",
+  "verticalAlign": "middle"
+}
+```
+
+---
+
+## Diamond
+
+**Best for:** Decision points, conditional branches
+
+### Properties
+
+```typescript
+{
+  type: "diamond",
+  text: "Valid?",
+  fontSize: 18,
+  textAlign: "center",
+  verticalAlign": "middle"
+}
+```
+
+### Use Cases
+
+| Scenario | Text Example |
+|----------|--------------|
+| **Yes/No decision** | "Is Valid?", "Exists?" |
+| **Multiple choice** | "Type?", "Status?" |
+| **Conditional** | "Score > 50?" |
+
+### Size Guidelines
+
+Diamonds need more space than rectangles for the same text:
+
+| Content | Width | Height |
+|---------|-------|--------|
+| Yes/No | 120-140px | 120-140px |
+| Short question | 160-180px | 160-180px |
+| Longer question | 200-220px | 200-220px |
+
+### Example
+
+```json
+{
+  "type": "diamond",
+  "x": 100,
+  "y": 100,
+  "width": 150,
+  "height": 150,
+  "backgroundColor": "#ffe4a3",
+  "text": "Valid?",
+  "fontSize": 18,
+  "textAlign": "center",
+  "verticalAlign": "middle"
+}
+```
+
+---
+
+## Arrow
+
+**Best for:** Flow direction, relationships, dependencies
+
+### Properties
+
+```typescript
+{
+  type: "arrow",
+  points: [[0, 0], [endX, endY]],  // Relative coordinates
+  roundness: { type: 2 },          // Curved
+  startBinding: null,              // Or { elementId, focus, gap }
+  endBinding: null
+}
+```
+
+### Arrow Directions
+
+#### Horizontal (Left to Right)
+
+```json
+{
+  "x": 100,
+  "y": 150,
+  "width": 200,
+  "height": 0,
+  "points": [[0, 0], [200, 0]]
+}
+```
+
+#### Vertical (Top to Bottom)
+
+```json
+{
+  "x": 200,
+  "y": 100,
+  "width": 0,
+  "height": 150,
+  "points": [[0, 0], [0, 150]]
+}
+```
+
+#### Diagonal
+
+```json
+{
+  "x": 100,
+  "y": 100,
+  "width": 200,
+  "height": 150,
+  "points": [[0, 0], [200, 150]]
+}
+```
+
+### Arrow Styles
+
+| Style | `strokeStyle` | `strokeWidth` | Use Case |
+|-------|---------------|---------------|----------|
+| **Normal flow** | `"solid"` | 2 | Standard connections |
+| **Optional/Weak** | `"dashed"` | 2 | Optional paths |
+| **Important** | `"solid"` | 3-4 | Emphasized flow |
+| **Dotted** | `"dotted"` | 2 | Indirect relationships |
+
+### Adding Arrow Labels
+
+Use separate text elements positioned near arrow midpoint:
+
+```json
+[
+  {
+    "type": "arrow",
+    "id": "arrow1",
+    "x": 100,
+    "y": 150,
+    "points": [[0, 0], [200, 0]]
+  },
+  {
+    "type": "text",
+    "x": 180,      // Near midpoint
+    "y": 130,      // Above arrow
+    "text": "sends",
+    "fontSize": 14
+  }
+]
+```
+
+---
+
+## Line
+
+**Best for:** Non-directional connections, dividers, borders
+
+### Properties
+
+```typescript
+{
+  type: "line",
+  points: [[0, 0], [x2, y2], [x3, y3], ...],
+  roundness: null  // Or { type: 2 } for curved
+}
+```
+
+### Use Cases
+
+| Scenario | Configuration |
+|----------|---------------|
+| **Divider** | Horizontal, thin stroke |
+| **Border** | Closed path (polygon) |
+| **Connection** | Multi-point path |
+| **Underline** | Short horizontal line |
+
+### Multi-Point Line Example
+
+```json
+{
+  "type": "line",
+  "x": 100,
+  "y": 100,
+  "points": [
+    [0, 0],
+    [100, 50],
+    [200, 0]
+  ]
+}
+```
+
+---
+
+## Text
+
+**Best for:** Labels, titles, annotations, standalone text
+
+### Properties
+
+```typescript
+{
+  type: "text",
+  text: "Label text",
+  fontSize: 20,
+  fontFamily: 1,        // 1=Virgil, 2=Helvetica, 3=Cascadia
+  textAlign: "left",
+  verticalAlign: "top"
+}
+```
+
+### Font Sizes by Purpose
+
+| Purpose | Font Size |
+|---------|-----------|
+| **Main title** | 28-36 |
+| **Section header** | 24-28 |
+| **Element label** | 18-22 |
+| **Annotation** | 14-16 |
+| **Small note** | 12-14 |
+
+### Width/Height Calculation
+
+```javascript
+// Approximate width
+const width = text.length * fontSize * 0.6;
+
+// Approximate height (single line)
+const height = fontSize * 1.2;
+
+// Multi-line
+const lines = text.split('\n').length;
+const height = fontSize * 1.2 * lines;
+```
+
+### Text Positioning
+
+| Position | textAlign | verticalAlign | Use Case |
+|----------|-----------|---------------|----------|
+| **Top-left** | `"left"` | `"top"` | Default labels |
+| **Centered** | `"center"` | `"middle"` | Titles |
+| **Bottom-right** | `"right"` | `"bottom"` | Footnotes |
+
+### Example: Title
+
+```json
+{
+  "type": "text",
+  "x": 100,
+  "y": 50,
+  "width": 400,
+  "height": 40,
+  "text": "System Architecture",
+  "fontSize": 32,
+  "fontFamily": 2,
+  "textAlign": "center",
+  "verticalAlign": "top"
+}
+```
+
+### Example: Annotation
+
+```json
+{
+  "type": "text",
+  "x": 150,
+  "y": 200,
+  "width": 100,
+  "height": 20,
+  "text": "User input",
+  "fontSize": 14,
+  "fontFamily": 1,
+  "textAlign": "left",
+  "verticalAlign": "top"
+}
+```
+
+---
+
+## Combining Elements
+
+### Pattern: Labeled Box
+
+```json
+[
+  {
+    "type": "rectangle",
+    "id": "box1",
+    "x": 100,
+    "y": 100,
+    "width": 200,
+    "height": 100,
+    "text": "Component",
+    "textAlign": "center",
+    "verticalAlign": "middle"
+  }
+]
+```
+
+### Pattern: Connected Boxes
+
+```json
+[
+  {
+    "type": "rectangle",
+    "id": "box1",
+    "x": 100,
+    "y": 100,
+    "width": 150,
+    "height": 80,
+    "text": "Step 1"
+  },
+  {
+    "type": "arrow",
+    "id": "arrow1",
+    "x": 250,
+    "y": 140,
+    "points": [[0, 0], [100, 0]]
+  },
+  {
+    "type": "rectangle",
+    "id": "box2",
+    "x": 350,
+    "y": 100,
+    "width": 150,
+    "height": 80,
+    "text": "Step 2"
+  }
+]
+```
+
+### Pattern: Decision Tree
+
+```json
+[
+  {
+    "type": "diamond",
+    "id": "decision",
+    "x": 100,
+    "y": 100,
+    "width": 140,
+    "height": 140,
+    "text": "Valid?"
+  },
+  {
+    "type": "arrow",
+    "id": "yes-arrow",
+    "x": 240,
+    "y": 170,
+    "points": [[0, 0], [60, 0]]
+  },
+  {
+    "type": "text",
+    "id": "yes-label",
+    "x": 250,
+    "y": 150,
+    "text": "Yes",
+    "fontSize": 14
+  },
+  {
+    "type": "rectangle",
+    "id": "yes-box",
+    "x": 300,
+    "y": 140,
+    "width": 120,
+    "height": 60,
+    "text": "Process"
+  }
+]
+```
+
+---
+
+## Summary
+
+| When you need... | Use this element |
+|------------------|------------------|
+| Process box | `rectangle` with text |
+| Decision point | `diamond` with question |
+| Flow direction | `arrow` |
+| Start/End | `ellipse` |
+| Title/Header | `text` (large font) |
+| Annotation | `text` (small font) |
+| Non-directional link | `line` |
+| Divider | `line` (horizontal) |
diff --git a/.claude/skills/excalidraw-diagram/references/excalidraw-schema.md b/.claude/skills/excalidraw-diagram/references/excalidraw-schema.md
new file mode 100644
index 0000000..900ff23
--- /dev/null
+++ b/.claude/skills/excalidraw-diagram/references/excalidraw-schema.md
@@ -0,0 +1,352 @@
+# Excalidraw JSON Schema Reference
+
+> **openmlsys note:** This file comes from github/awesome-copilot (MIT, see `../LICENSE.txt`) and is kept unchanged. Where it shows `fontFamily` 1 to 3, use `5` (Excalifont, or `3` for code) instead. House style and labels inside shapes are covered in [openmlsys-conventions.md](openmlsys-conventions.md).
+
+This document describes the structure of Excalidraw `.excalidraw` files for diagram generation.
+
+## Top-Level Structure
+
+```typescript
+interface ExcalidrawFile {
+  type: "excalidraw";
+  version: number;           // Always 2
+  source: string;            // "https://excalidraw.com"
+  elements: ExcalidrawElement[];
+  appState: AppState;
+  files: Record<string, any>; // Usually empty {}
+}
+```
+
+## AppState
+
+```typescript
+interface AppState {
+  viewBackgroundColor: string; // Hex color, e.g., "#ffffff"
+  gridSize: number;            // Typically 20
+}
+```
+
+## ExcalidrawElement Base Properties
+
+All elements share these common properties:
+
+```typescript
+interface BaseElement {
+  id: string;                  // Unique identifier
+  type: ElementType;           // See Element Types below
+  x: number;                   // X coordinate (pixels from top-left)
+  y: number;                   // Y coordinate (pixels from top-left)
+  width: number;               // Width in pixels
+  height: number;              // Height in pixels
+  angle: number;               // Rotation angle in radians (usually 0)
+  strokeColor: string;         // Hex color, e.g., "#1e1e1e"
+  backgroundColor: string;     // Hex color or "transparent"
+  fillStyle: "solid" | "hachure" | "cross-hatch";
+  strokeWidth: number;         // 1-4 typically
+  strokeStyle: "solid" | "dashed" | "dotted";
+  roughness: number;           // 0-2, controls hand-drawn effect (1 = default)
+  opacity: number;             // 0-100
+  groupIds: string[];          // IDs of groups this element belongs to
+  frameId: null;               // Usually null
+  index: string;               // Stacking order identifier
+  roundness: Roundness | null;
+  seed: number;                // Random seed for deterministic rendering
+  version: number;             // Element version (increment on edit)
+  versionNonce: number;        // Random number changed on edit
+  isDeleted: boolean;          // Should be false
+  boundElements: any;          // Usually null
+  updated: number;             // Timestamp in milliseconds
+  link: null;                  // External link (usually null)
+  locked: boolean;             // Whether element is locked
+}
+```
+
+## Element Types
+
+### Rectangle
+
+```typescript
+interface RectangleElement extends BaseElement {
+  type: "rectangle";
+  roundness: { type: 3 };      // 3 = rounded corners
+  text?: string;               // Optional text inside
+  fontSize?: number;           // Font size (16-32 typical)
+  fontFamily?: number;         // 1 = Virgil, 2 = Helvetica, 3 = Cascadia
+  textAlign?: "left" | "center" | "right";
+  verticalAlign?: "top" | "middle" | "bottom";
+}
+```
+
+**Example:**
+```json
+{
+  "id": "rect1",
+  "type": "rectangle",
+  "x": 100,
+  "y": 100,
+  "width": 200,
+  "height": 100,
+  "strokeColor": "#1e1e1e",
+  "backgroundColor": "#a5d8ff",
+  "text": "My Box",
+  "fontSize": 20,
+  "textAlign": "center",
+  "verticalAlign": "middle",
+  "roundness": { "type": 3 }
+}
+```
+
+### Ellipse
+
+```typescript
+interface EllipseElement extends BaseElement {
+  type: "ellipse";
+  text?: string;
+  fontSize?: number;
+  fontFamily?: number;
+  textAlign?: "left" | "center" | "right";
+  verticalAlign?: "top" | "middle" | "bottom";
+}
+```
+
+### Diamond
+
+```typescript
+interface DiamondElement extends BaseElement {
+  type: "diamond";
+  text?: string;
+  fontSize?: number;
+  fontFamily?: number;
+  textAlign?: "left" | "center" | "right";
+  verticalAlign?: "top" | "middle" | "bottom";
+}
+```
+
+### Arrow
+
+```typescript
+interface ArrowElement extends BaseElement {
+  type: "arrow";
+  points: [number, number][];  // Array of [x, y] coordinates relative to element
+  startBinding: Binding | null;
+  endBinding: Binding | null;
+  roundness: { type: 2 };      // 2 = curved arrow
+}
+```
+
+**Example:**
+```json
+{
+  "id": "arrow1",
+  "type": "arrow",
+  "x": 100,
+  "y": 100,
+  "width": 200,
+  "height": 0,
+  "points": [
+    [0, 0],
+    [200, 0]
+  ],
+  "roundness": { "type": 2 },
+  "startBinding": null,
+  "endBinding": null
+}
+```
+
+**Points explanation:**
+- First point `[0, 0]` is relative to `(x, y)`
+- Subsequent points are relative to the first point
+- For straight horizontal arrow: `[[0, 0], [width, 0]]`
+- For straight vertical arrow: `[[0, 0], [0, height]]`
+
+### Line
+
+```typescript
+interface LineElement extends BaseElement {
+  type: "line";
+  points: [number, number][];
+  startBinding: Binding | null;
+  endBinding: Binding | null;
+  roundness: { type: 2 } | null;
+}
+```
+
+### Text
+
+```typescript
+interface TextElement extends BaseElement {
+  type: "text";
+  text: string;
+  fontSize: number;
+  fontFamily: number;          // 1-3
+  textAlign: "left" | "center" | "right";
+  verticalAlign: "top" | "middle" | "bottom";
+  roundness: null;             // Text has no roundness
+}
+```
+
+**Example:**
+```json
+{
+  "id": "text1",
+  "type": "text",
+  "x": 100,
+  "y": 100,
+  "width": 150,
+  "height": 25,
+  "text": "Hello World",
+  "fontSize": 20,
+  "fontFamily": 1,
+  "textAlign": "left",
+  "verticalAlign": "top",
+  "roundness": null
+}
+```
+
+**Width/Height calculation:**
+- Width ≈ `text.length * fontSize * 0.6`
+- Height ≈ `fontSize * 1.2 * numberOfLines`
+
+## Bindings
+
+Bindings connect arrows to shapes:
+
+```typescript
+interface Binding {
+  elementId: string;           // ID of bound element
+  focus: number;               // -1 to 1, position along edge
+  gap: number;                 // Distance from element edge
+}
+```
+
+## Common Colors
+
+| Color Name | Hex Code | Use Case |
+|------------|----------|----------|
+| Black | `#1e1e1e` | Default stroke |
+| Light Blue | `#a5d8ff` | Primary entities |
+| Light Green | `#b2f2bb` | Process steps |
+| Yellow | `#ffd43b` | Important/Central |
+| Light Red | `#ffc9c9` | Warnings/Errors |
+| Cyan | `#96f2d7` | Secondary items |
+| Transparent | `transparent` | No fill |
+| White | `#ffffff` | Background |
+
+## ID Generation
+
+IDs should be unique strings. Common patterns:
+
+```javascript
+// Timestamp-based
+const id = Date.now().toString(36) + Math.random().toString(36).substr(2);
+
+// Sequential
+const id = "element-" + counter++;
+
+// Descriptive
+const id = "step-1", "entity-user", "arrow-1-to-2";
+```
+
+## Seed Generation
+
+Seeds are used for deterministic randomness in hand-drawn effect:
+
+```javascript
+const seed = Math.floor(Math.random() * 2147483647);
+```
+
+## Version and VersionNonce
+
+```javascript
+const version = 1;  // Increment when element is edited
+const versionNonce = Math.floor(Math.random() * 2147483647);
+```
+
+## Coordinate System
+
+- Origin `(0, 0)` is top-left corner
+- X increases to the right
+- Y increases downward
+- All units are in pixels
+
+## Recommended Spacing
+
+| Context | Spacing |
+|---------|---------|
+| Horizontal gap between elements | 200-300px |
+| Vertical gap between rows | 100-150px |
+| Minimum margin from edge | 50px |
+| Arrow-to-box clearance | 20-30px |
+
+## Font Families
+
+| ID | Name | Description |
+|----|------|-------------|
+| 1 | Virgil | Hand-drawn style (default) |
+| 2 | Helvetica | Clean sans-serif |
+| 3 | Cascadia | Monospace |
+
+## Validation Rules
+
+✅ **Required:**
+- All IDs must be unique
+- `type` must match actual element type
+- `version` must be an integer ≥ 1
+- `opacity` must be 0-100
+
+⚠️ **Recommended:**
+- Keep `roughness` at 1 for consistency
+- Use `strokeWidth` of 2 for clarity
+- Set `isDeleted` to `false`
+- Set `locked` to `false`
+- Keep `frameId`, `boundElements`, `link` as `null`
+
+## Complete Minimal Example
+
+```json
+{
+  "type": "excalidraw",
+  "version": 2,
+  "source": "https://excalidraw.com",
+  "elements": [
+    {
+      "id": "box1",
+      "type": "rectangle",
+      "x": 100,
+      "y": 100,
+      "width": 200,
+      "height": 100,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "#a5d8ff",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a0",
+      "roundness": { "type": 3 },
+      "seed": 1234567890,
+      "version": 1,
+      "versionNonce": 987654321,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "text": "Hello",
+      "fontSize": 20,
+      "fontFamily": 1,
+      "textAlign": "center",
+      "verticalAlign": "middle"
+    }
+  ],
+  "appState": {
+    "viewBackgroundColor": "#ffffff",
+    "gridSize": 20
+  },
+  "files": {}
+}
+```
diff --git a/.claude/skills/excalidraw-diagram/references/openmlsys-conventions.md b/.claude/skills/excalidraw-diagram/references/openmlsys-conventions.md
new file mode 100644
index 0000000..6a1b82c
--- /dev/null
+++ b/.claude/skills/excalidraw-diagram/references/openmlsys-conventions.md
@@ -0,0 +1,143 @@
+# openmlsys figure conventions
+
+These rules apply to every Excalidraw figure in the book. They extend
+`CONTRIBUTING/style_en.md`, section "Figure Guidelines".
+
+## Where files go
+
+| What | Path |
+|------|------|
+| Image folder (physical) | `v1/img/ch<NN>/` |
+| Same folder seen from a chapter | `v1/{en,zh}_chapters/img` → `../img`, `v2/{en,zh}_chapters/img` → `../../v1/img` (symlinks) |
+| Chapter file (v2) | `v2/en_chapters/<NN>_chapter_<topic>/<section>.md` |
+| Reference from a chapter | `../img/ch<NN>/<name>.svg` |
+
+- `NN` is the chapter number of the chapter that uses the figure, for example
+  `v2/en_chapters/03_chapter_accelerator/` → `img/ch03/`. v1 and v2 share this
+  folder, so run `ls v1/img/ch<NN>/` first to avoid name clashes.
+- Use lowercase names with hyphens: `pipeline-parallel-schedule`.
+- The source and the export share a stem. Commit both:
+
+  ```
+  v1/img/ch06/pipeline-parallel-schedule.excalidraw     # editable source (EN)
+  v1/img/ch06/pipeline-parallel-schedule.svg            # exported, used by en_chapters
+  v1/img/ch06/pipeline-parallel-schedule-zh.excalidraw  # editable source (ZH)
+  v1/img/ch06/pipeline-parallel-schedule-zh.svg         # exported, used by zh_chapters
+  ```
+- Do not save a source as `.json`. The repo `.gitignore` ignores `*.json`.
+
+## Export formats
+
+- **SVG** is the default for diagrams. Use **PNG** at 2x scale, at most
+  1200 px wide, only if the SVG renders badly (very large scenes, embedded
+  bitmaps).
+- **Never use PDF.** mdBook output is HTML, and browsers do not render
+  `![](x.pdf)` as an image. Some old v1 pages still point at `.pdf` files.
+  Replace those references when you redraw the figures. Do not copy the pattern.
+- Do not reference the `.excalidraw` file from Markdown.
+
+### How to export
+
+Use one of the following:
+
+1. **Excalidraw MCP.** The style guide recommends the official server at
+   `https://mcp.excalidraw.com` or the self-hosted `interactive-drawer`. Load
+   the scene and export it to SVG or PNG with the settings below.
+2. **excalidraw.com** (no install). Open the `.excalidraw` file, then
+   *Menu → Export image*. Set *Background* off, *Dark mode* off and scale 2x
+   for PNG, then save as SVG or PNG. Use *Menu → Save to…* to write the
+   `.excalidraw` file back after you edit it.
+3. **VS Code "Excalidraw" extension** (`pomdtr.excalidraw-editor`). It edits
+   `.excalidraw` files in the editor and exports through the same dialog.
+
+Export settings are the same for every method: light theme, no background
+(the style guide asks for figures without a white background), and
+`appState.exportWithDarkMode` false.
+
+## Dark mode
+
+The book's themes include `theme/dark-mode-images.css` (see
+`tests/test_dark_mode_images_css.py`). In the dark themes `navy`, `coal` and
+`ayu`, every body image gets a light gray `#e8e8e8` backdrop with 8 px
+padding. Images are never inverted. A transparent figure therefore sits on
+white in light mode and on `#e8e8e8` in dark mode. Consequences:
+
+- Text and strokes must be dark. Use `#1e1e1e` by default. White or pastel
+  text disappears, and `check_figures.py` reports it as an error.
+- Every shape needs a visible dark stroke. A very light fill (`#e9ecef`,
+  `#f8f9fa`) blends into the dark-mode backdrop, so do not rely on fill alone.
+- Do not export with Excalidraw's "dark mode" option.
+- Do not encode meaning by color alone. Add a label or a dashed/solid style
+  difference so the figure also works in grayscale print.
+
+## Palette
+
+These are Excalidraw's built-in swatches, so the figures stay editable from
+the stock UI.
+
+| Role | Fill (`backgroundColor`) | Stroke / text |
+|------|--------------------------|---------------|
+| Default component | `#a5d8ff` (blue) | `#1e1e1e` |
+| Data / tensors | `#b2f2bb` (green) | `#1e1e1e` |
+| Highlight / focus | `#ffec99` (yellow) | `#1e1e1e` |
+| Hardware / device | `#d0bfff` (violet) | `#1e1e1e` |
+| Error / bottleneck | `#ffc9c9` (red) | `#1e1e1e` |
+| Group / region frame | `transparent`, dashed stroke | `#495057` |
+| Accent arrows (optional) | – | `#1971c2`, `#2f9e44`, `#e03131`, `#f08c00` |
+
+Keep `fillStyle: "solid"`, `strokeWidth: 2`, `roughness: 1` and
+`roundness: {"type": 3}` for boxes. Use the same values across one chapter.
+
+## Text and fonts
+
+- Use `fontFamily: 5` (Excalifont) for all labels and `3` (Cascadia) for code
+  or identifiers. `1` (Virgil) is the legacy hand-drawn font and `2`
+  (Helvetica) depends on the reader's system fonts. Do not use either.
+- Excalidraw falls back to its bundled CJK hand-drawn font (Xiaolai) for
+  Chinese automatically, and SVG export embeds the glyphs it uses. You do not
+  need a different `fontFamily` for ZH variants.
+- Use `fontSize` 20 for box labels, 16 for edge labels and annotations, and
+  28 for an optional in-figure title. Most figures should have no title,
+  because the Markdown caption serves that purpose.
+- For math, write plain Unicode such as `W·x + b` or `∇L`. LaTeX is not
+  rendered inside Excalidraw.
+
+### Labels inside shapes (bound text)
+
+```json
+{ "id": "box1", "type": "rectangle", "...": "...",
+  "boundElements": [{ "type": "text", "id": "box1-label" }] },
+{ "id": "box1-label", "type": "text", "containerId": "box1",
+  "text": "Parameter Server", "originalText": "Parameter Server",
+  "fontSize": 20, "fontFamily": 5, "textAlign": "center",
+  "verticalAlign": "middle", "lineHeight": 1.25, "autoResize": true,
+  "strokeColor": "#1e1e1e", "...": "..." }
+```
+
+Center the text element in its container. Estimate the width as about
+0.6 × fontSize per Latin character and 1.0 × fontSize per CJK character.
+Excalidraw re-measures the text when the file is opened. Arrows that connect
+shapes should set `startBinding`/`endBinding` (`{"elementId": ..., "focus": 0, "gap": 4}`).
+The bound shapes must list the arrow in their `boundElements`.
+
+## Bilingual figures
+
+- If a figure contains words, make two variants: `<name>` (English, used by
+  `en_chapters`) and `<name>-zh` (Chinese, used by `zh_chapters`).
+- Build the ZH scene from the EN scene. Keep the ids and the geometry, and
+  translate only `text`/`originalText`. Chinese labels are often wider, so
+  widen the boxes and move neighbors symmetrically if needed.
+- Keep established English acronyms (GPU, NCCL, AllReduce, KV cache) in the ZH
+  variant. Follow `CONTRIBUTING/terminology.md` for the translated terms.
+- A figure with no words, or only symbols and math, uses one file shared by
+  both languages.
+- Captions belong in the Markdown in each language, not in the image.
+
+## Layout tips
+
+- Draw main flows left to right, or top to bottom for stacks and layers.
+- Leave a 40–80 px gap between boxes and use one box size per role.
+- Use about 20 elements per figure at most. Split a dense figure and refer
+  to both parts in the text.
+- Leave a margin of about 20 px around the content so the exported image
+  is not cropped tight.
diff --git a/.claude/skills/excalidraw-diagram/scripts/add-arrow.py b/.claude/skills/excalidraw-diagram/scripts/add-arrow.py
new file mode 100755
index 0000000..b23bc83
--- /dev/null
+++ b/.claude/skills/excalidraw-diagram/scripts/add-arrow.py
@@ -0,0 +1,317 @@
+#!/usr/bin/env python3
+"""
+Add arrows (connections) between elements in Excalidraw diagrams.
+
+From github/awesome-copilot (MIT, see ../LICENSE.txt); label font size raised to 16 for openmlsys.
+
+Usage:
+    python add-arrow.py <diagram_path> <from_x> <from_y> <to_x> <to_y> [OPTIONS]
+
+Options:
+    --style {solid|dashed|dotted}    Arrow line style (default: solid)
+    --color HEX                      Arrow color (default: #1e1e1e)
+    --label TEXT                     Add text label on the arrow
+    --use-edit-suffix                Edit via .excalidraw.edit to avoid editor overwrite issues (enabled by default; use --no-use-edit-suffix to disable)
+
+Examples:
+    python add-arrow.py diagram.excalidraw 300 200 500 300
+    python add-arrow.py diagram.excalidraw 300 200 500 300 --label "HTTP"
+    python add-arrow.py diagram.excalidraw 300 200 500 300 --style dashed --color "#7950f2"
+    python add-arrow.py diagram.excalidraw 300 200 500 300 --use-edit-suffix
+"""
+
+import json
+import sys
+import uuid
+from pathlib import Path
+from typing import Dict, Any
+
+
+def generate_unique_id() -> str:
+    """Generate a unique ID for Excalidraw elements."""
+    return str(uuid.uuid4()).replace('-', '')[:16]
+
+
+def prepare_edit_path(diagram_path: Path, use_edit_suffix: bool) -> tuple[Path, Path | None]:
+    """
+    Prepare a safe edit path to avoid editor overwrite issues.
+
+    Returns:
+        (work_path, final_path)
+        - work_path: file path to read/write during edit
+        - final_path: file path to rename back to (or None if not used)
+    """
+    if not use_edit_suffix:
+        return diagram_path, None
+
+    if diagram_path.suffix != ".excalidraw":
+        return diagram_path, None
+
+    edit_path = diagram_path.with_suffix(diagram_path.suffix + ".edit")
+
+    if diagram_path.exists():
+        if edit_path.exists():
+            raise FileExistsError(f"Edit file already exists: {edit_path}")
+        diagram_path.rename(edit_path)
+
+    return edit_path, diagram_path
+
+
+def finalize_edit_path(work_path: Path, final_path: Path | None) -> None:
+    """Finalize edit by renaming .edit back to .excalidraw if needed."""
+    if final_path is None:
+        return
+
+    if final_path.exists():
+        final_path.unlink()
+
+    work_path.rename(final_path)
+
+
+def create_arrow(
+    from_x: float,
+    from_y: float,
+    to_x: float,
+    to_y: float,
+    style: str = "solid",
+    color: str = "#1e1e1e",
+    label: str = None
+) -> list:
+    """
+    Create an arrow element.
+    
+    Args:
+        from_x: Starting X coordinate
+        from_y: Starting Y coordinate
+        to_x: Ending X coordinate
+        to_y: Ending Y coordinate
+        style: Line style (solid, dashed, dotted)
+        color: Arrow color
+        label: Optional text label on the arrow
+    
+    Returns:
+        List of elements (arrow and optional label)
+    """
+    elements = []
+    
+    # Arrow element
+    arrow = {
+        "id": generate_unique_id(),
+        "type": "arrow",
+        "x": from_x,
+        "y": from_y,
+        "width": to_x - from_x,
+        "height": to_y - from_y,
+        "angle": 0,
+        "strokeColor": color,
+        "backgroundColor": "transparent",
+        "fillStyle": "solid",
+        "strokeWidth": 2,
+        "strokeStyle": style,
+        "roughness": 1,
+        "opacity": 100,
+        "groupIds": [],
+        "frameId": None,
+        "index": "a0",
+        "roundness": {
+            "type": 2
+        },
+        "seed": 1000000000 + hash(f"{from_x}{from_y}{to_x}{to_y}") % 1000000000,
+        "version": 1,
+        "versionNonce": 2000000000 + hash(f"{from_x}{from_y}{to_x}{to_y}") % 1000000000,
+        "isDeleted": False,
+        "boundElements": [],
+        "updated": 1738195200000,
+        "link": None,
+        "locked": False,
+        "points": [
+            [0, 0],
+            [to_x - from_x, to_y - from_y]
+        ],
+        "startBinding": None,
+        "endBinding": None,
+        "startArrowhead": None,
+        "endArrowhead": "arrow",
+        "lastCommittedPoint": None
+    }
+    elements.append(arrow)
+    
+    # Optional label
+    if label:
+        mid_x = (from_x + to_x) / 2 - (len(label) * 5)
+        mid_y = (from_y + to_y) / 2 - 10
+        
+        label_element = {
+            "id": generate_unique_id(),
+            "type": "text",
+            "x": mid_x,
+            "y": mid_y,
+            "width": len(label) * 10,
+            "height": 20,
+            "angle": 0,
+            "strokeColor": color,
+            "backgroundColor": "transparent",
+            "fillStyle": "solid",
+            "strokeWidth": 2,
+            "strokeStyle": "solid",
+            "roughness": 1,
+            "opacity": 100,
+            "groupIds": [],
+            "frameId": None,
+            "index": "a0",
+            "roundness": None,
+            "seed": 1000000000 + hash(label) % 1000000000,
+            "version": 1,
+            "versionNonce": 2000000000 + hash(label) % 1000000000,
+            "isDeleted": False,
+            "boundElements": [],
+            "updated": 1738195200000,
+            "link": None,
+            "locked": False,
+            "text": label,
+            "fontSize": 16,
+            "fontFamily": 5,
+            "textAlign": "center",
+            "verticalAlign": "top",
+            "containerId": None,
+            "originalText": label,
+            "autoResize": True,
+            "lineHeight": 1.25
+        }
+        elements.append(label_element)
+    
+    return elements
+
+
+def add_arrow_to_diagram(
+    diagram_path: Path,
+    from_x: float,
+    from_y: float,
+    to_x: float,
+    to_y: float,
+    style: str = "solid",
+    color: str = "#1e1e1e",
+    label: str = None
+) -> None:
+    """
+    Add an arrow to an Excalidraw diagram.
+    
+    Args:
+        diagram_path: Path to the Excalidraw diagram file
+        from_x: Starting X coordinate
+        from_y: Starting Y coordinate
+        to_x: Ending X coordinate
+        to_y: Ending Y coordinate
+        style: Line style (solid, dashed, dotted)
+        color: Arrow color
+        label: Optional text label
+    """
+    print(f"Creating arrow from ({from_x}, {from_y}) to ({to_x}, {to_y})")
+    arrow_elements = create_arrow(from_x, from_y, to_x, to_y, style, color, label)
+    
+    if label:
+        print(f"  With label: '{label}'")
+    
+    # Load diagram
+    print(f"Loading diagram: {diagram_path}")
+    with open(diagram_path, 'r', encoding='utf-8') as f:
+        diagram = json.load(f)
+    
+    # Add arrow elements
+    if 'elements' not in diagram:
+        diagram['elements'] = []
+    
+    original_count = len(diagram['elements'])
+    diagram['elements'].extend(arrow_elements)
+    print(f"  Added {len(arrow_elements)} elements (total: {original_count} -> {len(diagram['elements'])})")
+    
+    # Save diagram
+    print(f"Saving diagram")
+    with open(diagram_path, 'w', encoding='utf-8') as f:
+        json.dump(diagram, f, indent=2, ensure_ascii=False)
+    
+    print(f"✓ Successfully added arrow to diagram")
+
+
+def main():
+    """Main entry point."""
+    if hasattr(sys.stdout, "reconfigure"):
+        # Ensure consistent UTF-8 output on Windows consoles.
+        sys.stdout.reconfigure(encoding="utf-8")
+    if len(sys.argv) < 6:
+        print("Usage: python add-arrow.py <diagram_path> <from_x> <from_y> <to_x> <to_y> [OPTIONS]")
+        print("\nOptions:")
+        print("  --style {solid|dashed|dotted}    Line style (default: solid)")
+        print("  --color HEX                      Color (default: #1e1e1e)")
+        print("  --label TEXT                     Text label on arrow")
+        print("  --use-edit-suffix                Edit via .excalidraw.edit to avoid editor overwrite issues (enabled by default; use --no-use-edit-suffix to disable)")
+        print("\nExamples:")
+        print("  python add-arrow.py diagram.excalidraw 300 200 500 300")
+        print("  python add-arrow.py diagram.excalidraw 300 200 500 300 --label 'HTTP'")
+        sys.exit(1)
+    
+    diagram_path = Path(sys.argv[1])
+    from_x = float(sys.argv[2])
+    from_y = float(sys.argv[3])
+    to_x = float(sys.argv[4])
+    to_y = float(sys.argv[5])
+    
+    # Parse optional arguments
+    style = "solid"
+    color = "#1e1e1e"
+    label = None
+    # Default: use edit suffix to avoid editor overwrite issues
+    use_edit_suffix = True
+    
+    i = 6
+    while i < len(sys.argv):
+        if sys.argv[i] == '--style':
+            if i + 1 < len(sys.argv):
+                style = sys.argv[i + 1]
+                if style not in ['solid', 'dashed', 'dotted']:
+                    print(f"Error: Invalid style '{style}'. Must be: solid, dashed, or dotted")
+                    sys.exit(1)
+                i += 2
+            else:
+                print("Error: --style requires an argument")
+                sys.exit(1)
+        elif sys.argv[i] == '--color':
+            if i + 1 < len(sys.argv):
+                color = sys.argv[i + 1]
+                i += 2
+            else:
+                print("Error: --color requires an argument")
+                sys.exit(1)
+        elif sys.argv[i] == '--label':
+            if i + 1 < len(sys.argv):
+                label = sys.argv[i + 1]
+                i += 2
+            else:
+                print("Error: --label requires a text argument")
+                sys.exit(1)
+        elif sys.argv[i] == '--use-edit-suffix':
+            use_edit_suffix = True
+            i += 1
+        elif sys.argv[i] == '--no-use-edit-suffix':
+            use_edit_suffix = False
+            i += 1
+        else:
+            print(f"Error: Unknown option: {sys.argv[i]}")
+            sys.exit(1)
+    
+    # Validate inputs
+    if not diagram_path.exists():
+        print(f"Error: Diagram file not found: {diagram_path}")
+        sys.exit(1)
+    
+    try:
+        work_path, final_path = prepare_edit_path(diagram_path, use_edit_suffix)
+        add_arrow_to_diagram(work_path, from_x, from_y, to_x, to_y, style, color, label)
+        finalize_edit_path(work_path, final_path)
+    except Exception as e:
+        print(f"Error: {e}")
+        sys.exit(1)
+
+
+if __name__ == '__main__':
+    main()
diff --git a/.claude/skills/excalidraw-diagram/scripts/check_figures.py b/.claude/skills/excalidraw-diagram/scripts/check_figures.py
new file mode 100755
index 0000000..7c5fa05
--- /dev/null
+++ b/.claude/skills/excalidraw-diagram/scripts/check_figures.py
@@ -0,0 +1,146 @@
+#!/usr/bin/env python3
+"""Lint Excalidraw sources and figure references for the openmlsys book.
+
+Standard library only (Python 3.8+). No network access.
+
+Usage:
+    python3 check_figures.py PATH [PATH ...]
+
+PATH may be:
+  * a ``.excalidraw`` file or a directory (searched recursively) -- each scene
+    is checked for structural problems and house-style violations, and for an
+    exported ``.svg``/``.png`` sibling with the same stem;
+  * a ``.md`` chapter file -- every image reference is checked: it must not
+    point at a ``.pdf`` and the referenced file must exist.
+
+Exit status is 1 if any ERROR was reported, otherwise 0 (WARNINGs only).
+"""
+
+from __future__ import annotations
+
+import json
+import re
+import sys
+from pathlib import Path
+
+ALLOWED_FONTS = {5, 3}  # 5 = Excalifont (body text), 3 = Cascadia (code)
+MIN_FONT_SIZE = 16
+IMAGE_REF = re.compile(r"!\[[^\]]*\]\(([^)\s]+)(?:\s+\"[^\"]*\")?\)")
+
+errors = 0
+warnings = 0
+
+
+def report(level: str, where: Path, msg: str) -> None:
+    global errors, warnings
+    if level == "ERROR":
+        errors += 1
+    else:
+        warnings += 1
+    print(f"{level}: {where}: {msg}")
+
+
+def luminance(color: str) -> float | None:
+    m = re.fullmatch(r"#([0-9a-fA-F]{6})", color or "")
+    if not m:
+        return None
+    r, g, b = (int(m.group(1)[i : i + 2], 16) / 255 for i in (0, 2, 4))
+    return 0.2126 * r + 0.7152 * g + 0.0722 * b
+
+
+def check_scene(path: Path) -> None:
+    try:
+        doc = json.loads(path.read_text(encoding="utf-8"))
+    except (OSError, ValueError) as exc:
+        report("ERROR", path, f"not valid JSON: {exc}")
+        return
+    if doc.get("type") != "excalidraw" or not isinstance(doc.get("elements"), list):
+        report("ERROR", path, 'expected {"type": "excalidraw", "elements": [...]}')
+        return
+
+    elements = [e for e in doc["elements"] if not e.get("isDeleted")]
+    by_id = {}
+    for el in elements:
+        eid = el.get("id")
+        if not eid:
+            report("ERROR", path, f"element without id: {el.get('type')}")
+        elif eid in by_id:
+            report("ERROR", path, f"duplicate id {eid!r}")
+        by_id[eid] = el
+
+    for el in elements:
+        eid, etype = el.get("id"), el.get("type")
+        tag = f"{etype} {eid!r}"
+        if etype != "text" and "text" in el:
+            report("ERROR", path, f"{tag} has a 'text' property; Excalidraw ignores it. "
+                   "Use a separate text element with containerId (see references/openmlsys-conventions.md)")
+        if etype == "text":
+            if el.get("fontFamily") not in ALLOWED_FONTS:
+                report("WARNING", path, f"{tag} uses fontFamily {el.get('fontFamily')}; house style is 5 (or 3 for code)")
+            if (el.get("fontSize") or 0) < MIN_FONT_SIZE:
+                report("WARNING", path, f"{tag} fontSize {el.get('fontSize')} < {MIN_FONT_SIZE}; may be illegible when scaled")
+            lum = luminance(el.get("strokeColor", ""))
+            if lum is not None and lum > 0.6:
+                report("ERROR", path, f"{tag} text color {el['strokeColor']} is too light for the light page / #e8e8e8 dark-mode backdrop")
+            cid = el.get("containerId")
+            if cid:
+                container = by_id.get(cid)
+                if container is None:
+                    report("ERROR", path, f"{tag} containerId {cid!r} does not exist")
+                elif not any(b.get("id") == eid for b in container.get("boundElements") or []):
+                    report("ERROR", path, f"container {cid!r} does not list text {eid!r} in boundElements")
+        for b in el.get("boundElements") or []:
+            if b.get("id") not in by_id:
+                report("ERROR", path, f"{tag} boundElements references missing {b.get('id')!r}")
+        for key in ("startBinding", "endBinding"):
+            binding = el.get(key)
+            if binding and binding.get("elementId") not in by_id:
+                report("ERROR", path, f"{tag} {key} references missing {binding.get('elementId')!r}")
+        if etype in ("arrow", "line") and len(el.get("points") or []) < 2:
+            report("ERROR", path, f"{tag} needs at least two points")
+
+    app = doc.get("appState") or {}
+    if app.get("exportWithDarkMode"):
+        report("WARNING", path, "exportWithDarkMode is on; export in light mode (the site handles dark mode)")
+
+    stem = path.name[: -len(".excalidraw")]
+    if not any((path.parent / f"{stem}{ext}").exists() for ext in (".svg", ".png")):
+        report("WARNING", path, f"no exported {stem}.svg or {stem}.png next to the source")
+
+
+def check_markdown(path: Path) -> None:
+    text = path.read_text(encoding="utf-8")
+    for lineno, line in enumerate(text.splitlines(), 1):
+        for target in IMAGE_REF.findall(line):
+            if re.match(r"[a-z]+://", target):
+                continue
+            where = f"{path}:{lineno}"
+            if target.lower().endswith(".pdf"):
+                report("ERROR", Path(where), f"{target} is a PDF; browsers cannot show it in mdBook, export SVG/PNG instead")
+            if target.endswith(".excalidraw"):
+                report("ERROR", Path(where), f"{target} is an Excalidraw source; reference the exported SVG/PNG")
+            if not (path.parent / target).exists():
+                report("ERROR", Path(where), f"{target} does not resolve relative to the chapter file")
+
+
+def main(argv: list[str]) -> int:
+    if not argv or argv[0] in ("-h", "--help"):
+        print(__doc__)
+        return 0 if argv else 2
+    for arg in argv:
+        p = Path(arg)
+        if p.is_dir():
+            for scene in sorted(p.rglob("*.excalidraw")):
+                check_scene(scene)
+        elif p.suffix == ".excalidraw":
+            check_scene(p)
+        elif p.suffix == ".md":
+            check_markdown(p)
+        else:
+            report("ERROR", p, "not a directory, .excalidraw or .md file")
+    print(f"{errors} error(s), {warnings} warning(s)")
+    return 1 if errors else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv[1:]))
diff --git a/.claude/skills/excalidraw-diagram/templates/data-flow-diagram-template.excalidraw b/.claude/skills/excalidraw-diagram/templates/data-flow-diagram-template.excalidraw
new file mode 100644
index 0000000..84c981f
--- /dev/null
+++ b/.claude/skills/excalidraw-diagram/templates/data-flow-diagram-template.excalidraw
@@ -0,0 +1,414 @@
+{
+  "type": "excalidraw",
+  "version": 2,
+  "source": "https://excalidraw.com",
+  "elements": [
+    {
+      "id": "external-entity-1",
+      "type": "rectangle",
+      "x": 100,
+      "y": 200,
+      "width": 120,
+      "height": 80,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "#ffc9c9",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a0",
+      "roundness": {
+        "type": 3
+      },
+      "seed": 1001001001,
+      "version": 1,
+      "versionNonce": 2002002002,
+      "isDeleted": false,
+      "boundElements": [
+        {
+          "type": "text",
+          "id": "external-entity-1-label"
+        }
+      ],
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false
+    },
+    {
+      "id": "external-entity-1-label",
+      "type": "text",
+      "x": 138.4,
+      "y": 228.75,
+      "width": 43.199999999999996,
+      "height": 22.5,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a0V",
+      "roundness": null,
+      "seed": 1001001008,
+      "version": 1,
+      "versionNonce": 2002002009,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "text": "User",
+      "fontSize": 18,
+      "fontFamily": 5,
+      "textAlign": "center",
+      "verticalAlign": "middle",
+      "containerId": "external-entity-1",
+      "originalText": "User",
+      "autoResize": true,
+      "lineHeight": 1.25
+    },
+    {
+      "id": "data-flow-1",
+      "type": "arrow",
+      "x": 220,
+      "y": 240,
+      "width": 80,
+      "height": 0,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a1",
+      "roundness": {
+        "type": 2
+      },
+      "seed": 1001001002,
+      "version": 1,
+      "versionNonce": 2002002003,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "points": [
+        [
+          0,
+          0
+        ],
+        [
+          80,
+          0
+        ]
+      ],
+      "startBinding": null,
+      "endBinding": null
+    },
+    {
+      "id": "flow-label-1",
+      "type": "text",
+      "x": 230,
+      "y": 220,
+      "width": 80,
+      "height": 20,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a2",
+      "roundness": null,
+      "seed": 1001001003,
+      "version": 1,
+      "versionNonce": 2002002004,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "text": "input data",
+      "fontSize": 16,
+      "fontFamily": 5,
+      "textAlign": "left",
+      "verticalAlign": "top"
+    },
+    {
+      "id": "process-1",
+      "type": "ellipse",
+      "x": 300,
+      "y": 200,
+      "width": 120,
+      "height": 80,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "#a5d8ff",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a3",
+      "roundness": null,
+      "seed": 1001001004,
+      "version": 1,
+      "versionNonce": 2002002005,
+      "isDeleted": false,
+      "boundElements": [
+        {
+          "type": "text",
+          "id": "process-1-label"
+        }
+      ],
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false
+    },
+    {
+      "id": "process-1-label",
+      "type": "text",
+      "x": 326.4,
+      "y": 220.0,
+      "width": 67.2,
+      "height": 40.0,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a3V",
+      "roundness": null,
+      "seed": 1001001011,
+      "version": 1,
+      "versionNonce": 2002002012,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "text": "Process\nData",
+      "fontSize": 16,
+      "fontFamily": 5,
+      "textAlign": "center",
+      "verticalAlign": "middle",
+      "containerId": "process-1",
+      "originalText": "Process\nData",
+      "autoResize": true,
+      "lineHeight": 1.25
+    },
+    {
+      "id": "data-flow-2",
+      "type": "arrow",
+      "x": 420,
+      "y": 240,
+      "width": 80,
+      "height": 0,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a4",
+      "roundness": {
+        "type": 2
+      },
+      "seed": 1001001005,
+      "version": 1,
+      "versionNonce": 2002002006,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "points": [
+        [
+          0,
+          0
+        ],
+        [
+          80,
+          0
+        ]
+      ],
+      "startBinding": null,
+      "endBinding": null
+    },
+    {
+      "id": "flow-label-2",
+      "type": "text",
+      "x": 425,
+      "y": 220,
+      "width": 100,
+      "height": 20,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a5",
+      "roundness": null,
+      "seed": 1001001006,
+      "version": 1,
+      "versionNonce": 2002002007,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "text": "processed data",
+      "fontSize": 16,
+      "fontFamily": 5,
+      "textAlign": "left",
+      "verticalAlign": "top"
+    },
+    {
+      "id": "data-store-1",
+      "type": "rectangle",
+      "x": 500,
+      "y": 200,
+      "width": 150,
+      "height": 80,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "#96f2d7",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a6",
+      "roundness": null,
+      "seed": 1001001007,
+      "version": 1,
+      "versionNonce": 2002002008,
+      "isDeleted": false,
+      "boundElements": [
+        {
+          "type": "text",
+          "id": "data-store-1-label"
+        }
+      ],
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false
+    },
+    {
+      "id": "data-store-1-label",
+      "type": "text",
+      "x": 527.0,
+      "y": 220.0,
+      "width": 96.0,
+      "height": 40.0,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a6V",
+      "roundness": null,
+      "seed": 1001001014,
+      "version": 1,
+      "versionNonce": 2002002015,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "text": "Data Store\n(Database)",
+      "fontSize": 16,
+      "fontFamily": 5,
+      "textAlign": "center",
+      "verticalAlign": "middle",
+      "containerId": "data-store-1",
+      "originalText": "Data Store\n(Database)",
+      "autoResize": true,
+      "lineHeight": 1.25
+    },
+    {
+      "id": "data-store-line",
+      "type": "line",
+      "x": 500,
+      "y": 225,
+      "width": 150,
+      "height": 0,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a7",
+      "roundness": null,
+      "seed": 1001001008,
+      "version": 1,
+      "versionNonce": 2002002009,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "points": [
+        [
+          0,
+          0
+        ],
+        [
+          150,
+          0
+        ]
+      ],
+      "startBinding": null,
+      "endBinding": null
+    }
+  ],
+  "appState": {
+    "viewBackgroundColor": "#ffffff",
+    "gridSize": 20
+  },
+  "files": {}
+}
diff --git a/.claude/skills/excalidraw-diagram/templates/flowchart-template.excalidraw b/.claude/skills/excalidraw-diagram/templates/flowchart-template.excalidraw
new file mode 100644
index 0000000..f1c67e2
--- /dev/null
+++ b/.claude/skills/excalidraw-diagram/templates/flowchart-template.excalidraw
@@ -0,0 +1,312 @@
+{
+  "type": "excalidraw",
+  "version": 2,
+  "source": "https://excalidraw.com",
+  "elements": [
+    {
+      "id": "step1",
+      "type": "rectangle",
+      "x": 400,
+      "y": 200,
+      "width": 200,
+      "height": 80,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "#b2f2bb",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a0",
+      "roundness": {
+        "type": 3
+      },
+      "seed": 1234567890,
+      "version": 1,
+      "versionNonce": 987654321,
+      "isDeleted": false,
+      "boundElements": [
+        {
+          "type": "text",
+          "id": "step1-label"
+        }
+      ],
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false
+    },
+    {
+      "id": "step1-label",
+      "type": "text",
+      "x": 464.0,
+      "y": 227.5,
+      "width": 72.0,
+      "height": 25.0,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a0V",
+      "roundness": null,
+      "seed": 1234567897,
+      "version": 1,
+      "versionNonce": 987654328,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "text": "Step 1",
+      "fontSize": 20,
+      "fontFamily": 5,
+      "textAlign": "center",
+      "verticalAlign": "middle",
+      "containerId": "step1",
+      "originalText": "Step 1",
+      "autoResize": true,
+      "lineHeight": 1.25
+    },
+    {
+      "id": "arrow1",
+      "type": "arrow",
+      "x": 500,
+      "y": 280,
+      "width": 0,
+      "height": 100,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "transparent",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a1",
+      "roundness": {
+        "type": 2
+      },
+      "seed": 1234567891,
+      "version": 1,
+      "versionNonce": 987654322,
+      "isDeleted": false,
+      "boundElements": null,
+      "updated": 1706659200000,
+      "link": null,
+      "locked": false,
+      "points": [
+        [
+          0,
+          0
+        ],
+        [
+          0,
+          100
+        ]
+      ],
+      "startBinding": null,
+      "endBinding": null
+    },
+    {
+      "id": "step2",
+      "type": "rectangle",
+      "x": 400,
+      "y": 380,
+      "width": 200,
+      "height": 80,
+      "angle": 0,
+      "strokeColor": "#1e1e1e",
+      "backgroundColor": "#b2f2bb",
+      "fillStyle": "solid",
+      "strokeWidth": 2,
+      "strokeStyle": "solid",
+      "roughness": 1,
+      "opacity": 100,
+      "groupIds": [],
+      "frameId": null,
+      "index": "a2",
+      "roundness": {
+        "type": 3
+      },
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index 0000000..2a16b64
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index 0000000..8ae4122
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diff --git a/CONTRIBUTING/CONTRIBUTING_en.md b/CONTRIBUTING/CONTRIBUTING_en.md
index df84fa9..e75c7d7 100644
--- a/CONTRIBUTING/CONTRIBUTING_en.md
+++ b/CONTRIBUTING/CONTRIBUTING_en.md
@@ -78,6 +78,27 @@ After modifying source files under `zh_chapters/` or `en_chapters/`, the browser
 
 For detailed custom syntax (math, figure labels, citations, code blocks) and figure guidelines, refer to the **[Writing Style Guide](style_en.md)**.
 
+### Drawing Figures with AI Agents (Excalidraw Skill)
+
+The repository includes a project-level agent skill,
+[`.claude/skills/excalidraw-diagram/`](../.claude/skills/excalidraw-diagram/SKILL.md).
+`.agents/skills/excalidraw-diagram` is a symlink to the same directory.
+Claude Code, and other agents that support the Agent Skills format, load it
+automatically when you ask them to draw a figure for the book.
+
+The skill tells the agent to:
+
+- keep the editable `.excalidraw` source next to the exported SVG/PNG in `v1/img/ch<NN>/`
+- make separate `-zh` variants for figures that contain text
+- follow a palette that stays legible in dark mode
+- never reference PDF images
+
+Lint figures and chapter references before you commit:
+
+```bash
+python3 .claude/skills/excalidraw-diagram/scripts/check_figures.py v1/img/ch06 v2/en_chapters/06_chapter_training_systems/index.md
+```
+
 ---
 
 ## Submission Workflow
diff --git a/CONTRIBUTING/CONTRIBUTING_zh.md b/CONTRIBUTING/CONTRIBUTING_zh.md
index acee9de..306371f 100644
--- a/CONTRIBUTING/CONTRIBUTING_zh.md
+++ b/CONTRIBUTING/CONTRIBUTING_zh.md
@@ -78,6 +78,26 @@ cd v2/config/en_config && mdbook serve --open
 
 详细的自定义语法（公式、图片标签、文献引用、代码块等）及图片规范，请参阅 **[写作样式规范](style_zh.md)**。
 
+### 使用 AI Agent 绘图（Excalidraw Skill）
+
+仓库内置了项目级 Agent Skill
+[`.claude/skills/excalidraw-diagram/`](../.claude/skills/excalidraw-diagram/SKILL.md)。
+`.agents/skills/excalidraw-diagram` 是指向同一目录的符号链接。
+让 Claude Code 或其他支持 Agent Skills 格式的 Agent 为本书绘图时，它们会自动加载该 Skill。
+
+该 Skill 要求 Agent：
+
+- 将可编辑的 `.excalidraw` 源文件与导出的 SVG/PNG 一起放在 `v1/img/ch<NN>/`
+- 为含文字的图另做 `-zh` 中文版本
+- 使用在暗色模式下同样清晰的配色
+- 不引用 PDF 图片
+
+提交前可运行以下命令检查图片源文件与章节中的引用：
+
+```bash
+python3 .claude/skills/excalidraw-diagram/scripts/check_figures.py v1/img/ch06 v2/zh_chapters/06_chapter_training_systems/index.md
+```
+
 ---
 
 ## 提交流程