Drop a picture or video onto a composition track (#419)

A track item has always carried an object, and an object needs an
asset. Instead of objects without one, a dropped picture or video gets
a sprite the composition owns, the way a song owns its sounds (#355).
To the rest of the code it is a sprite like any other.

- The asset list starts a drag for textures, vector art, pixel art and
  videos. Dropped on a track, they make the sprite and the item in one
  undo step. A video item is as long as the video and follows it.
- One sprite per picture and composition. A second drop of the same
  video shares it. Once no item wears it, it goes in the same step.
- Deleting a composition takes its sprites along. A duplicate gets
  sprites of its own.
- Pasting into a scene or container turns the sprite into a project
  asset. Pasting into another composition gives that one a copy. The
  clipboard carries the sprite, so cut and paste loses nothing.
- Owned sprites stay out of the asset list and the pickers. The
  inspector shows them with a dot in front.
- "Make project asset" in the item's menu, on the timeline and in the
  hierarchy, turns the sprite into an ordinary one.
- The first track of the Composition tab gets its top line, as the
  Song tab's has.

Found on the way: loading kept an owner only for the asset types a
song can hold. A composition's sprite would have lost its owner and
turned up in the list after a reload.

Tests:
- Unit composition-sprites: 11 new. Without the rule 5 of them are
  red, without the loading fix the loading test is.
- e2e 69 on melektaus-atx: ALL PASSED, 15 checks, 69k new
- Gate on melektaus-atx: 305 files, 4714 tests

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

tools/e2e/69-composition-tab.mjs

@@ -5,7 +5,8 @@
  * past it. An animation dragged in becomes a clip that moves the item it overlaps in the picture. The hierarchy lists what is alive at the playhead, Delete
  * takes an item with its object, and Play runs the playhead to the end. The video of an item about
  * to begin is opened ahead, so the cut to it shows its picture at once, and Play waits for a video
- * whose player is still loading. A copied item is pasted under the pointer or at the playhead.
+ * whose player is still loading. A copied item is pasted under the pointer or at the playhead. A
+ * video dragged in wears a sprite the composition keeps to itself.
  *
  * Everything it adds carries a `ct69` id and goes again at the end.
  */
@@ -308,8 +309,47 @@ export async function compositionTab(t) {
     JSON.stringify({ copies, picked: pickedNow?.objectIds }),
   );
 
+  // 69k. A video dragged straight onto a track (#419) is an item as long as the video, wearing a sprite
+  // the composition keeps and the Assets panel leaves out; a second drop shares it, and undoing both
+  // takes the sprite along.
+  const kept = (id) =>
+    json(`(() => {
+      const p = __gm.state.project;
+      const c = p.assets.find((a) => a.id === '${CUT}');
+      const object = c.objects.find((o) => o.id === ${JSON.stringify(id)});
+      const sprite = object && p.assets.find((a) => a.id === object.assetId);
+      return { item: c.items.find((i) => i.id === ${JSON.stringify(id)}) ?? null,
+        sprite: sprite ? { id: sprite.id, type: sprite.type, owner: sprite.owner, texture: sprite.props.texture } : null,
+        owned: p.assets.filter((a) => a.owner === '${CUT}').length };
+    })()`);
+  const videoDrop = await dragAsset(VIDEO, await json(`__gm.composition.laneAt('ct69-t2', 70)`));
+  const one = await kept((await json('__gm.state.selection'))?.objectIds?.[0] ?? '');
+  const row = one.sprite ? await json(`__gm.assetRow('${one.sprite.id}')`) : 'none';
+  await dragAsset(VIDEO, await json(`__gm.composition.laneAt('ct69-t2', 100)`));
+  const two = await kept((await json('__gm.state.selection'))?.objectIds?.[0] ?? '');
+  await evaluate('__gm.state.undo()');
+  await evaluate('__gm.state.undo()');
+  const undone = await kept('');
+  check(
+    'a video dropped on a track wears a sprite the composition keeps, shared by a second drop',
+    videoDrop?.what === 'item' &&
+      videoDrop.rejection === '' &&
+      one.item?.track === 'ct69-t2' &&
+      one.item.start === 70 &&
+      one.item.length === 20 &&
+      one.sprite?.type === 'sprite' &&
+      one.sprite.owner === CUT &&
+      one.sprite.texture === VIDEO &&
+      row === null &&
+      two.item?.start === 100 &&
+      two.sprite?.id === one.sprite.id &&
+      two.owned === 1 &&
+      undone.owned === 0,
+    JSON.stringify({ videoDrop, one, row, two, undone }),
+  );
+
   await showTab('scene');
   await mutate(`
     const p = __gm.state.project;
-    p.assets = p.assets.filter((a) => !a.id.startsWith('ct69-'));`);
+    p.assets = p.assets.filter((a) => !a.id.startsWith('ct69-') && !a.owner?.startsWith('ct69-'));`);
 }