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.
