fix(preview): flag unplayable media when previewing a standalone asset

[ctrlaltking]

Summary

• VideoEngine.previewAsset() builds AVPlayerItem(url: asset.url) directly with no error handling. If the source file can't actually decode (corrupt, truncated, unsupported codec variant), the preview just sits blank/frozen with no feedback anywhere — no log, no UI state change.
• This detection already exists for the timeline composition path: CompositionBuilder probes each clip's tracks and records failures into offlineMediaRefs/unprocessableMediaRefs, which PreviewContainerView uses to show a "Couldn't Prepare Media" overlay with an explanation and a report button. That path never ran for an asset previewed standalone (before being placed on the timeline).
• Reproduced the failure mode directly: AVURLAsset(url:).loadTracks(withMediaType: .video) on a garbage/corrupt file throws AVErrorFailedDependenciesKey "Cannot Open" — exactly the case that silently fails today.

Fix

After swapping in the player item in previewAsset(), probe the asset's tracks in the background. On failure, insert the asset id into editor.unprocessableMediaRefs, reusing the existing offline/unprocessable UI instead of leaving a silent blank player.

Test plan

• swift build — clean build
• swift test — full suite passes (one pre-existing failure in CompositionBuilderTests.fractionalSpeedAudioUsesTruncatedSourceFramesForCompositionInsertion confirmed present on unmodified main too, unrelated to this change)
• Verified AVURLAsset.loadTracks throws on a corrupt file with a video extension, matching the failure mode this fix detects

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Note

Low Risk
Small, preview-only change that toggles existing editor UI state; no auth, export, or timeline composition logic is modified.

Overview
Standalone media previews no longer fail silently when AVPlayerItem(url:) cannot decode the file. After swapping in the player item in previewAsset(), the engine asynchronously probes AVURLAsset.loadTracks(withMediaType:) for the correct audio or video track.

If decoding fails or no track is found, the asset id is added to editor.unprocessableMediaRefs (with a preview log); if the file becomes playable, the id is removed. That reuses the same unprocessable/offline overlay the timeline composition path already drives via CompositionBuilder, which never ran for library-only asset preview.

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Stale media probe race

Medium Severity

Each call to verifyAssetPlayable starts a new unstructured Task without cancelling earlier probes. openPreviewTab always calls activateTab, so re-opening the same asset can run overlapping loadTracks checks. A slower, older task can finish last and update unprocessableMediaRefs after a newer probe, showing or hiding the unplayable overlay incorrectly.

 

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