Consolidate EPUB TTS segment planning around a single canonical windowing model.
Introduce `epub-canonical-window.ts` to provide stable, viewport-independent
segment windows for both playback and sidebar. Refactor segment planning,
prefetching, and playback handoff to use canonical ordinals and keys, ensuring
that segments straddling page breaks are uniquely identified and never replayed
across pages. Update TTSContext, SegmentsSidebar, and related hooks to support
windowed canonical segments and precise ordinal continuity. Extend types and
interfaces to carry canonical segment metadata and windowing context.
Add comprehensive unit tests for canonical window selection, segment
materialization, and cache behavior.
BREAKING CHANGE: EPUB TTS segment planning, playback, and sidebar now rely on
canonical windowing, affecting segment key generation and playback logic.
cb9ca37500