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>

src/editor/inspector/compositionSection.ts

@@ -1,10 +1,17 @@
+import { MakeProjectAssetCommand } from '../../core/commands/assetCommands';
 import { removeFromComposition, SetCompositionCommand } from '../../core/commands/compositionCommands';
 import type { CompositionAsset, CompositionItem } from '../../core/model/Asset';
-import { clipLength, compositionFps, fittedLength, itemVideo } from '../../core/model/composition';
+import {
+  clipLength,
+  compositionFps,
+  fittedLength,
+  isCompositionSprite,
+  itemVideo,
+} from '../../core/model/composition';
 import { resizedItem } from '../../core/model/compositionEdit';
-import { findAssetOf, type Project } from '../../core/model/Project';
+import { findAsset, findAssetOf, type Project } from '../../core/model/Project';
 import type { PropertyDef } from '../../core/properties/PropertySchema';
-import { Button, formatNumber } from '../../ui';
+import { Button, formatNumber, type MenuEntry } from '../../ui';
 import { createFieldRow } from './fieldFactory';
 import { addGap, addNameField, addNote, addTitle, type InspectorHost } from './host';
 import { assetLink } from './objectSection';
@@ -215,6 +222,25 @@ export function fitToMedia(host: Pick<InspectorHost, 'state'>, holderId: string,
     host.state.execute(new SetCompositionCommand(holderId, { items }, 'Fit to media'));
 }
 
+/**
+ * For an item wearing a sprite its composition keeps (#419): that sprite turned into a project asset,
+ * listed and placeable anywhere. Greyed out for any other.
+ */
+export function makeProjectAssetEntry(
+  host: Pick<InspectorHost, 'state'>,
+  holderId: string,
+  objectId: string,
+): MenuEntry {
+  const { project } = host.state;
+  const object = findAssetOf(project, holderId, 'composition')?.objects.find((o) => o.id === objectId);
+  const asset = object && findAsset(project, object.assetId);
+  return {
+    label: 'Make project asset',
+    disabled: !asset || !isCompositionSprite(asset),
+    onSelect: () => asset && host.state.execute(new MakeProjectAssetCommand(asset.id)),
+  };
+}
+
 function withOffset(item: CompositionItem, offset: number): CompositionItem {
   const { fitMedia: _, ...rest } = item;
   return { ...rest, offset: Number.isFinite(offset) ? offset : 0 };