feat: add v1/v2 versioning with language selector (#494)

* feat: add v1/v2 versioning and language selector for mdbook

- Copy current content to v1/ directory (1st Edition)
- Create v2/ directory with new TOC structure (2nd Edition) and placeholder chapters
- Add version selector (V1/V2) and language toggle (EN/ZH) in top-right nav bar
- Add build scripts: build_mdbook_v1.sh, build_mdbook_v2.sh
- Update assemble_docs_publish_tree.py to support v1/v2 deployment layout
- Fix mdbook preprocessor to use 'sections' key (v0.4.43 compatibility)
- Update .gitignore for new build artifact directories
- Deployment layout: / = v2 EN, /cn/ = v2 ZH, /v1/ = v1 EN, /v1/cn/ = v1 ZH

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* build: update CI to build and verify all four books (v1/v2 x EN/ZH)

- Clarify step names: "Build v2 (EN + ZH)" and "Build v1 (EN + ZH)"
- Add verification step to check all four index.html outputs exist
- Deploy workflow assembles: / = v2 EN, /cn/ = v2 ZH, /v1/ = v1 EN, /v1/cn/ = v1 ZH

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: gracefully skip missing TOC entries instead of crashing

resolve_toc_target() now returns None for missing files instead of
raising FileNotFoundError. This fixes v1 EN build where chapter index
files reference TOC entry names that don't match actual filenames.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

theme/version-selector.js

@@ -0,0 +1,74 @@
+// Version and Language selector for OpenMLSys mdbook
+(function () {
+  "use strict";
+
+  var path = window.location.pathname;
+
+  // Detect current version and language from URL
+  var currentVersion = "v2";
+  var currentLang = "en";
+
+  if (path.match(/\/v1(\/|$)/)) {
+    currentVersion = "v1";
+  }
+  if (path.match(/\/cn(\/|$)/)) {
+    currentLang = "zh";
+  }
+
+  // Build base paths
+  function basePath(version, lang) {
+    var docsRoot = path.replace(/\/v1\/.*/, "/").replace(/\/cn\/.*/, "/");
+    docsRoot = docsRoot.replace(/(\/docs\/?).*/, "/docs/");
+    if (!docsRoot.endsWith("/")) docsRoot += "/";
+
+    var p = docsRoot;
+    if (version === "v1") p += "v1/";
+    if (lang === "zh") p += "cn/";
+    return p;
+  }
+
+  var container = document.createElement("span");
+  container.className = "openmlsys-nav-selectors";
+
+  // --- Version links: V1 | V2 ---
+  var versions = [
+    { label: "V1", value: "v1" },
+    { label: "V2", value: "v2" },
+  ];
+
+  versions.forEach(function (v) {
+    var a = document.createElement("a");
+    a.className = "openmlsys-selector-link";
+    a.textContent = v.label;
+    a.href = basePath(v.value, currentLang);
+    if (v.value === currentVersion) a.classList.add("active");
+    container.appendChild(a);
+  });
+
+  // Separator
+  var sep = document.createElement("span");
+  sep.className = "openmlsys-selector-sep";
+  container.appendChild(sep);
+
+  // --- Language toggle: single button that switches to the other language ---
+  var otherLang = currentLang === "zh" ? "en" : "zh";
+  var langLink = document.createElement("a");
+  langLink.className = "openmlsys-selector-link";
+  langLink.textContent = currentLang === "zh" ? "EN" : "ZH";
+  langLink.href = basePath(currentVersion, otherLang);
+  container.appendChild(langLink);
+
+  // Insert into .right-buttons, before existing icons
+  function insertSelector() {
+    var rightButtons = document.querySelector(".right-buttons");
+    if (rightButtons) {
+      rightButtons.insertBefore(container, rightButtons.firstChild);
+    }
+  }
+
+  if (document.readyState === "loading") {
+    document.addEventListener("DOMContentLoaded", insertSelector);
+  } else {
+    insertSelector();
+  }
+})();