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>

README.md

@@ -47,7 +47,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2047 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2048 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs