GoTalk NOW .gotalk-book share exports: nested layout, image resolution, format routing

[willwade]

Summary

Real-world GoTalk NOW books shared from the app arrive as .gotalk-book ZIP
exports (often .zip-suffixed) — a layout the processor previously rejected
outright. This PR teaches the processor both container variants and fixes the
image/semantic gaps found testing against real share exports.

Container support

• Nested .gotalk-book layout: plists live under <BookName>.gotalk-book/
  rather than the ZIP root (.gtbz backups); __MACOSX/ resource-fork entries
  are ignored. processTexts round-trips the nested structure in place
  (nothing hoisted/flattened).
• Format routing: MyBook.gotalk-book.zip and .gotalk-book names route
  to the GoTalk processor in getProcessor (Node + browser); .gotalk-book
  added to supported extensions; core/analyze accepts a gotalk-book key;
  the CLI content-sniffs bare .zip archives for GoTalk plists.

Image resolution (previously resolved nothing on real files)

• Bundled images matched by basename, case-insensitive, with extension
  guessing — Location/QuickRecoveryPath are iOS container paths that
  never match archive entry names verbatim.
• Entries from the app's bundled "GoTalk Image Library" clip-art are
  skipped; the QuickRecoveryPath entry is preferred (second entry when
  multiple); final fallback is the pre-rendered snapshot
  <page>-<button>-<buttonCount>.png.
• Symbol libraries detected generically (metacom/pcs/symbolstix/widgit)
  and marked without resolving a bundled file.

Semantics

• JumpTo sentinels (0, -1, empty) no longer produce phantom navigation
  targets.
• Sparse pages without ButtonCount sized by highest button index + 1;
  non-square counts derive a near-square rectangle (cols = ⌈√n⌉, rows = ⌈n/cols⌉).

Testing

• 9 new tests: nested-layout load, basename+extension image resolution,
  clip-art skipping, snapshot fallback, library-symbol marking, JumpTo
  sentinels, sparse grid sizing, nested processTexts round-trip, book-suffix
  routing in both entrypoints, CLI .gotalk-book.zip + bare-.zip sniffing.
• Full suite: 91 suites / 837 tests pass; lint and type-check clean.
• All fixtures are synthetic (built in test code) — no user data.