Ignore invalid transcript timestamps

[happykawayigt]

Summary

• ignore transcript nodes whose data-m value cannot be parsed, preventing them from poisoning playhead binary search
• return before changing transcript state when clicks target whitespace or malformed timed nodes
• keep malformed nodes in a clean, inert visual state
• add regression tests for both invalid timestamp paths

Validation

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

Fixes #262