Remove legacy ffmpeg frame fallback

[adilei]

Summary

• remove the system-ffmpeg fallback used only by recordings predating captured source frames
• use captured-frame manifests exclusively and skip frame enrichment gracefully when they are unavailable
• add a session-level regression proving video-only sessions never attempt ffmpeg discovery and still produce their bundle and description

Why remove instead of time out

Captured-frame manifests have shipped in every tagged release since v0.1.0. Keeping the fallback would add timeout, process-kill, partial-output cleanup, and test-injection machinery solely for unreleased development-era sessions. Removing it eliminates both the indefinite hang in #14 and the orphaned partial-output path tracked in #11 (L6).

Testing

• targeted extractor and pipeline tests: 8 passed
• npm run typecheck
• npm test: 176 passed
• npm run build

Closes #14.