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>

tools/mdbook_preprocessor.py

@@ -43,7 +43,7 @@ def main() -> int:
             for key, fields in parse_bib(extra_bib).items():
                 bib_db.setdefault(key, fields)
 
-    chapters = iter_chapters(book.get("items", []))
+    chapters = iter_chapters(book.get("sections") or book.get("items") or [])
 
     # Pass 1: collect all :label: directives and figure labels
     ref_label_map: dict[str, str] = {}