Load the next item's video before a composition cuts to it (#417)

Two sprites with videos back to back left a blank frame at the cut.
The video player only came into being once the item began, and a
sprite is not drawn until its video has a first frame. So the second
item was missing until its video had loaded.

- compositionAhead lists the items that begin within the next second.
  Near the end of a looping composition it also lists the items at
  its start.
- An item that is alive already is not loaded ahead. Two times would
  otherwise pull at the same player.
- The frame carries these videos hidden and paused on their first
  picture at the item's offset. Their players get the id the item
  will have, so the cut reuses them instead of opening a second one.
- This works the same in the tab's preview and in the game.
- One second of lead (AHEAD_SECONDS) is an assumption. It can go up
  if large videos still flash.

Tests:
- Unit composition: 27 green, 2 new. Unit composition-play: 14
  green, 1 new for the path in the game.
- e2e 69: ALL PASSED, 12 checks. The new 69h puts two video items
  back to back and checks the second player is open and paused at
  its offset before the cut, and drawn in the same redraw as the
  cut.
- Without loading ahead in the preview 69h fails with cut: null.
  Bending two spots in the model turned their unit tests red.
- Gate on melektaus-atx: CHECK EXIT 0, 302 files, 4665 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, 2032 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2033 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