Slice 21 — Audio-driven duration + full-sequence media loop

[cbaucom]

Summary

• totalFrames = max(visual pass duration, total audio duration) when clips are present; visual wins when longer.
• Planner loops the full included slide sequence with correct transitions at loop boundaries and Ken Burns continuing across passes.
• Partial tail truncates the last slide when audio ends mid-sequence.
• Composition uses startFrame-scoped keys for repeated slide IDs in TransitionSeries.

Test plan

• pnpm test — 246 tests pass
• pnpm lint && pnpm build
• Playlist longer than one slide pass → preview runs until audio ends with repeating media
• Audio shorter than slides → show length unchanged
• Beat sync on → nudged durations repeat each loop pass

Closes #46

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