Wait for a composition's videos and copy and paste its items (#417)

A composition lost the first frames of a video item. Its clock ran
while the player was still loading, in the tab and in the game. The
game only waited for videos once, when it started. A composition that
a behavior starts later spawned its items after that wait.

- The view now records which video sprites it left out for lack of a
  first picture, one entry per object.
- The composition tab holds its playhead while the preview misses a
  video, for at most VIDEO_WAIT_MS (3 s) like the game.
- CompositionSystem holds a composition's clock while one of its live
  items misses its video. The rest of the game keeps running.
- The editor play and the published game pass their view to the run
  as RunServices.pictures.
- VIDEO_WAIT_MS moves to core/model/video.ts, because the runtime
  does not import from viewport/.
- Videos loaded ahead for coming items do not hold the clock.

Items can now be copied and pasted inside the composition tab.

- Ctrl+C keeps the timing of the copied items next to the objects.
- Ctrl+V pastes objects and items in one undo step. The earliest item
  lands at the frame under the pointer, on the track there.
- Without the pointer over the tracks it lands at the playhead, on the
  selected track or on the top track of the selected items.
- Several items keep their distances and tracks to each other.
- The same copy pastes into a scene as plain objects. Objects copied
  from a scene do not paste into a composition, they have no timing.

Tests:
- Unit composition-edit, clipboard, composition-play: 3 new for
  pasting, 1 new for the clock waiting in the game
- e2e 69i (play from the end waits for the video), 69j (paste under
  the pointer and at the playhead), 68d (a composition started from
  On start waits for its video)
- each new check is red without its fix
- README: 2036 editor checks

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, 2033 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2036 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