refactor(toc): cache navigable structure per book (#3869)

* refactor(toc): cache TOC + section fragments per book

Moves the TOC regrouping and section-fragment computation out of
foliate-js/epub.js #updateSubItems into the readest client as
computeBookNav / hydrateBookNav in utils/toc.ts. The result is
persisted to Books/{hash}/nav.json — capturing the book's full
navigable structure (TOC hierarchy + sections with hierarchical
fragments). Compute once, persist locally, hydrate on subsequent
opens. Designed to serve current human-facing navigation (TOC
sidebar, progress math) and future agentic navigation (LLM-driven
seeking by structural location).

Versioned by BOOK_NAV_VERSION for forward invalidation. Existing
books regenerate transparently on next open.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: update worktree scripts

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/readest-app/src/utils/book.ts

@@ -35,6 +35,9 @@ export const getCoverFilename = (book: Book) => {
 export const getConfigFilename = (book: Book) => {
   return `${book.hash}/config.json`;
 };
+export const getBookNavFilename = (book: Book) => {
+  return `${book.hash}/nav.json`;
+};
 export const isBookFile = (filename: string) => {
   return Object.values(EXTS).includes(filename.split('.').pop()!);
 };