feat: add comprehensive test coverage for 7 modules
New test files (150 tests, 0 failures):
- subtitles/srt: 22 tests (timestamps, multi-line, malformed, line endings)
- subtitles/ass: 17 tests (styles, alignment, override tags, resolution)
- subtitles/parse: 8 tests (dispatch, case-insensitive, unsupported ext)
- utils/math-extended: 35 tests (clamp, snapToStep, isNearlyEqual, format)
- fps-extended: 16 tests (frameRate conversion, NTSC, GCD reduction)
- rendering: 15 tests (transform builder, opacity, blend modes)
- params: 32 tests (coerce, valueKind, interpolation, numeric range)
Also includes HANDOFF.md with full documentation of workflow,
completed work, and next steps for other agents.
f247641703