fix(core,player,studio): bound trimmed audio playback to the clip window

[vanceingalls]

Problem

Trimmed audio clips kept playing past the clip edge — the audio ran on to the source file's natural end even though the timeline clip ended earlier. This happened across every audio path, so fixing one didn't fix the symptom in the Studio.

Root cause

Three independent paths each ignored the clip's authored window (data-duration / media-start):

1. Parent audio proxy (<hyperframes-player> when iframe autoplay is blocked) — the proxy played to the source end and never re-read live trims.
2. Runtime element gating (core/runtime) — the duration resolver computed clip length only from source-file length capped by host-composition duration, never consulting the element's own data-duration.
3. WebAudio transport — the audible path in the Studio (the iframe <audio> is muted while WebAudio plays the decoded buffer). schedulePlayback called AudioBufferSourceNode.start(when, offset) with no duration, so the buffer played from the trim offset to the file's natural end regardless of the clip length.

Confirmed live: audioOwner: "runtime", the iframe <audio> was muted: true + paused: true at its trimmed edge (element gating worked), but WebAudio kept the buffer running — so the trim never bound the audible output.

Fix

• WebAudio (webAudioTransport.ts + init.ts): schedulePlayback now takes the clip's data-duration and passes a bounded duration to start(when, offset, duration), so the buffer stops at the trimmed edge. Past-end schedules are skipped.
• Runtime element gating (init.ts): the duration resolver caps each clip by min(sourceLength, hostWindow, ownDataDuration), so a trimmed <audio>/<video> element pauses at its edge.
• Parent proxy (parent-media.ts): the proxy re-reads its source clip's live data-start/data-duration each tick and pauses outside the clip window, resuming on re-entry.

Studio trim UX (same root area)

• Resize (useTimelineEditing.ts): a start-edge drag now live-patches the media-start/playback-start offset to the iframe, so dragging the left edge trims into the source instead of only repositioning the clip.
• Waveform (AudioWaveform.tsx + useRenderClipContent.ts): the rendered peaks are windowed to the trimmed slice [mediaStart, mediaStart + duration·rate], so the waveform tracks the clip edges instead of squeezing the whole file into the clip width.

Tests

• webAudioTransport.test.ts: 5 new cases for the clip-duration bound (in-progress, future, past-end skip, rate scaling, unbounded-when-omitted).
• parent-media.test.ts: pause past trimmed end; re-read live data-duration.

Verification

In Studio: trim the music clip, play past the trimmed edge → audio now goes silent at the edge (previously ran to the source end). Untrimmed playback unchanged.

🤖 Generated with Claude Code

[vanceingalls]

• feat(studio,cli): music beat detection with timeline guides + headless beats CLI #1424
• fix(core,player,studio): bound trimmed audio playback to the clip window #1430 👈 (View in Graphite)
• main

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