feat(videos): swap-clip drawer — pick a different clip in a beat slot

[jjackson]

Summary

@jjackson: "I want to be able to switch a video to a different one in the template library."

The backend op (set-clip-asset) and the frontend op type already shipped — this wires up the UI to dispatch it.

What changed

• New ClipPickerPanel renders every alias in spec.manifest as a thumbnail-tile grid. Current alias is sorted first with a "Current" badge. Click a different one → dispatch set-clip-asset → drawer closes. Existing trim values on the slot are preserved.
• ClipSlotWidget header gets a ↔ Swap button next to ✏ Edit trim. Both stop propagation so the card's catch-all onClick (which still opens trim) doesn't double-fire.
• WidgetRef union extended with a clip-picker variant; EditDrawer routes to the new panel.

Tested

On chc/run-001 Field-footage clip 1:

• 6 Swap buttons on the page (3 scene-clip + 3 product-beat slots)
• Picker opens with the 8 manifest aliases as a 2-column grid + video thumbnails
• Current alias has the "Current" badge
• Clicking @mobile-learn → first slot's header updates from @field-group-around-woman to @mobile-learn (dispatched as a set-clip-asset op into the dirty buffer; saves with the next batch like any other edit)

Future

• Browse the wider video library (more than the 8 manifest entries) and auto-add to the manifest on pick — the workspace has dozens of clips across mobile-screencast / web-screencast / field-broll; today's picker is limited to whatever a particular spec already references.
• Hover-preview that plays the muted clip (Chrome's autoplay-on-hover heuristics are spicy; deferred).

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