[cbaucom]

Summary

Planner now emits a ramped soundtrack ducking envelope around video slides (20% hold level, 6-frame attack/release). Composition applies envelope keyframes to Html5Audio and resolved videoVolume to Video. Slide settings popover adds per-slide audio overrides: mute music, mute video audio, and custom mix levels.

Acceptance criteria

• Video between photos ducks music during the clip and recovers with ramped keyframes (not stepped)
• muteVideoAudio keeps music at full level; muteMusic silences soundtrack for the slide
• Property test: duck segments exactly cover unmuted video-audio spans

Test plan

• pnpm test
• pnpm lint
• pnpm build
• Playwright smoke: open slide settings → audio override controls visible
• Manual: folder with video + soundtrack → confirm audible duck during video playback

Code review

Decision: APPROVE (4/5)

Non-blocking follow-ups: colocated volumeAtFrame tests; music slider shows 100% while auto-ducking until overridden.

Closes #12

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