Add 《源氏物语》as a second grown tree in 我的树

[cubxxw]

Summary

• Extracts the per-book tree data (branches, annotations, echo) out of MyTree.tsx into a new src/books.ts, so a "book" is a plain data record and the page component carries no book-specific content.
• Adds a second complete themed book: 《源氏物语》(紫式部, ~1008) alongside the existing 《道德经》— five 帖 from 桐壺 to 夢浮橋, each with an anchoring line, 入木/青芽/读过 states, a cinnabar signature, one continuation thread ("陆声 · 2 年后接续"), and its own 周年回声 (anniversary echo).
• Adds a quiet book switcher below the reader's name (title + author/era subtitle, underlined when active) that swaps which tree is drawn in place — no routing needed.
• Updates README.md and APP_NOTES.md to document the multi-book data model and how to add another themed book.

Test plan

• npm run build (type-check + production build) passes
• Manually verified in a headless browser: both trees draw on correctly, all five 源氏物语 branches/leaves/labels render, the thread and hover caption work, and switching between 道德经 ↔ 源氏物语 resets the active branch and shows the correct planting date / echo per book

🤖 Generated with Claude Code

https://claude.ai/code/session_01AotmBqgJa7s8hM7rwAG6vD

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Generated by Claude Code

[cubxxw]

Superseded by #10, which plants 源氏物語 (and 心經) directly onto the shelf using main's current Book shape.

Since this PR was opened, main has moved on: PR #1 merged 金剛經 and with it a richer src/books/types.ts (ShelfEntry + BioNode + BioRing + ReadingChapter + StrataLayer + CompareSpan) plus src/pages/Bookshelf, BookDetail, Reading. The flat src/books.ts file and the tree-only Book shape here can't ride on top of that.

What was salvaged into #10: the intent — turn the 源氏物語 ghost in comingSoon into a live spine. The tree structure here is preserved on main under the shared MyTree geometry; the biography/reading content comes from #7's richer 12 nodes and my own draft.

Closing in favor of #10. Thanks for the push toward getting a second tree on the shelf.