Replace EPUB-specific word highlight mapping with shared token-sequence alignment
logic used by HTML and PDF viewers. Remove bespoke alignment code and tests in
favor of a single primitive (`locateAlignmentWordSpans`) that robustly maps
spoken words to rendered text regions, tolerant of transcription and formatting
differences. Update EPUB highlighting to cache per-segment word-region spans for
efficient re-use. Add unit tests for the new alignment logic.
Enhance EPUB TTS canonical windowing by guarding against stale async completions
during rapid page turns, preventing outdated segment/highlight state from
overwriting the active page. Update SegmentsSidebar to robustly handle errors
in canonicalization, ensuring sidebar state is cleared on failure and avoiding
residual mappings from previous pages. Refine canonical window documentation to
clarify non-overlapping segment partitioning per page. Add a language-aware
cache test to verify correct canonical plan isolation across languages.
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.
