Edit a composition's tracks, items and clips in its own tab (#417)

A composition could be placed and played, but not filled. Its tab
puts the picture at the playhead above a timeline of tracks, so a cut
or a cutscene is laid out by dragging assets onto tracks.

- The timeline takes the song tab's geometry with frames for ticks:
  ruler in seconds, scrubbing, play and pause, + Track. Zoom and pan
  glide through viewTween like the other tabs.
- A placeable asset dropped on a track becomes an item, 3 s long or
  as long as its sprite's video. Dropped on an item it lands behind.
- Items move without overlapping. The right edge stops at the next
  item. The left edge trims into a video and not before its start.
- An animation dropped on a track becomes a narrower clip bar above
  the item it plays on. The asset panel now starts a drag for
  animations too.
- The picture is a scene view over compositionHolder. A click picks
  an item or its child, a drag moves the real object. It takes no
  assets, since an object there would have no item.
- The hierarchy lists the items alive at the playhead. Reparenting is
  off there, items stay at the top level.
- The inspector shows a TIMING block on an item (start, length,
  offset for a video, Fit to media) and edits tracks and clips.
- Delete, the context menu and Remove item take item and object in
  one undo step.
- fitMedia: an override that gives an item another video sets its
  length in the same undo step, capped at the next item. Dragging an
  edge or typing a length turns it off.
- A composition inside a composition is refused.

Tests:
- Unit composition-edit (new): 19 tests. Four counter-checks each
  turned their test red. The "back to 3 s" test got a step in
  between after one stayed green.
- e2e 69 (new): 11 checks, 9 own. The hierarchy check reads the rows
  shown, after a counter-check on the model's rows stayed green.
- Gate on melektaus-atx: CHECK EXIT 0, 302 files, 4662 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, 2023 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2032 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