Drag clips of videos and sounds straight onto a track (#425)

A clip of a video could not be laid on a composition track at all, and
neither could a clip of a sound, although #418 and the wiki said so.
The only place a drag starts is the asset panel, and it listed assets,
not clips. The texture label of an item playing a video clip was dead
too, because the reference lookup did not know video clips.

- The asset panel gives a sound or video with clips a chevron of its
  own. It unfolds one row per clip and starts closed.
- A clip row drags like an asset. One click shows the clip in the
  inspector without leaving the tab (`EditorState.glance`). A double
  click opens its sound or video with the clip picked.
- A video clip dropped on a composition track becomes an item as long
  as the clip. It wears the sprite of its whole video, so a composition
  keeps one sprite per video, and the clip is a texture override on
  the object. Sprite, item and override are one undo step
  (`droppedPicture`).
- A sound clip becomes an audio item as long as the clip. The song tab
  takes sound clips on the same way, through `findSound`.
- `referencedTarget` lets a reference label link to the clip itself.
  One click pings the clip row, a double click opens the asset with the
  clip picked. The Plays row of an audio item reads "Sound / Clip".
- `clipOwner` in `sound.ts` finds the sound or video a clip id belongs
  to. `activateAssetOrClip` uses it for the double clicks.

Found on the way: the Blip of the e2e seed is only 0.25 s long, so a
clip of 0.2 to 0.7 s on it is cut to one frame. 69s takes the Drone
(0.75 s) instead.

Tests:
- tsc, eslint, Prettier clean
- Unit on stein2 (composition, references, sound, video, editor):
  94 files, 1238 tests green
- e2e 69 on melektaus-atx: ALL PASSED, 25 checks, 2 of them new (69s)
- Gate on melektaus-atx with the fix for 68k on top: CHECK EXIT 0,
  313 files, 4841 tests
- Counter checks, all red: no clip rows in e2e 69 (no drag starts),
  no clip override in the unit test, the link aimed at the asset
  instead of the clip
- Not covered by its own e2e check: sound clips on the song tab

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/composition-edit.test.ts

@@ -472,8 +472,8 @@ describe('items and their objects', () => {
     const cut = makeCut();
     const screen = new CreateAssetCommand('sprite', 'Screen', { texture: cut.videoId });
     cut.stack.execute(screen);
-    expect(droppedLength(cut.project, cut.comp(), screen.asset)).toBe(300);
-    expect(droppedLength(cut.project, cut.comp(), findAsset(cut.project, cut.boxId)!)).toBe(180);
+    expect(droppedLength(cut.project, cut.comp(), screen.asset.id)).toBe(300);
+    expect(droppedLength(cut.project, cut.comp(), cut.boxId)).toBe(180);
   });
 });