fix(editor): cascade clip modifier deletion - #242

This is the strongest PR in the batch, and the reason is a path neither the description nor CodeRabbit names.

The old guard was oldClips.length > 0 && newClips.length > 0. The first half was dead — arr.length !== oldClips.length already implies oldClips.length >= 1. The second half is the bug: delete the last clip and every anchored zoom, annotation, speed and camera region survives with clipId still pointing at the clip that just went away, and useTimeline.ts:977 writes that straight to disk.

It doesn't stay dangling. Reopen the project: Preview mounts the asset even with an empty timeline, handleLoadedMetadata sees clips.length === 0 and calls replaceTimeline to seed a full-duration clip (NewEditorShell.tsx:364-380). reconcileRegionsAfterReplace then finds those orphans anchored to a clip that isn't in surviving, falls through to anchorRegionsWithDerivedMs, and re-ventilates them from their stale ruler ms onto the new clip. placed comes back true, so they're kept. The zooms the user deleted reappear, at whatever source time their old ms happens to land on. Your change is what stops that.

I re-checked this against the region/pill work that landed since your merge base — no interaction. Clip deletion is only reachable from the per-clip trash button, which renders only while the clip is selected, and selectClip clears the pill selection first.

Merging. Two notes inline, neither blocking.

Threads cleared — the isAnchored asymmetry is unreachable on current main and the multi-asset coverage gap is tracked in #249, neither worth holding this for.

Approving. This is the one I most want in the tree: deleting the last clip left every anchored zoom, annotation, speed and camera region pointing at a clip that no longer existed, and the next project open reseeded a clip and re-ventilated them onto new footage. Deleted pills coming back on top of different video is about as bad as an editor bug gets, and the two new tests fail on main and pass here.