Avoid busy polling after timed transcript ends

[happykawayigt]

Summary

• fall back to a 250 ms status poll after playback passes the final timed word
• use the same bounded fallback when the next data-m value is missing, invalid, or already behind the playhead
• add timer regression tests for the end-of-transcript and invalid-timestamp paths

Validation

• npm run build
• npm test -- --runInBand (76 tests passed)
• git diff --check

Fixes #263