Add synchronized media conversion and guarded rough cuts

[RichardBray]

Source-time rebuilds currently serialize separate screen/camera/audio sources, and reuse the first clip on an already-cut track, potentially discarding later adjustments. This adds synchronized source groups and refuses timelines whose existing edits cannot be preserved.

• Add rebuild --sync-groups with explicit source offsets, shared cut boundaries, unique clip IDs, finite/range/frame-rounding validation, and member timings in dry-run output.
• Add convert-media to create a new editable MP4/WAV .cmproj from one uncut TREC. Preserve static framing/gain and track flags, validate streams, refuse existing output, and leave the original project and Camtasia settings/cache untouched. Native cursor data and unused bin items are explicitly excluded. A bounded final-frame hold covers short decoded TREC tails.
• Request full local Whisper JSON for word offsets and exclude reversed word timestamps without losing segment text.
• Document supported input and limitations. Existing rebuild lock/backup protections remain; --force does not bypass timeline validation.

Validation: 80 tests pass (including an FFmpeg conversion integration test); TypeScript check and standalone CLI build pass. A separate-media sample was opened, played, saved/reopened and exported in Camtasia 2026.2.1; exported audio/video durations matched. Full-recording validation also passed: an 864.5-second recording converted and rebuilt into 34 synchronized sections (102 clips), played and saved/reopened in Camtasia 2026.2.1, and exported as a 319.5-second MP4 with matching video/audio duration. Full export decode passed, sampled compositions were inspected, and exported-audio correlation against the edited PCM showed offsets below 0.5 ms at five points. This is a workflow mitigation related to #17, not a claim to fix Camtasia's underlying decoder issue.