Silence a composition video as soon as its item ends

A video in a composition plays on a driven player that the item tells
where to stand every frame. When the item ended, its object was gone
and stopped asking, but the player ran on with its sound until it was
let go for being unused. That took 2 to 4 seconds, and in a pause it
never happened at all. The same held for the composition preview.

- A driven player nobody asked for in 100 ms is now paused and muted.
  When its wearer comes back it carries on where it should be, as
  before. It is still let go after 2 to 4 seconds.
- A player still without its first picture keeps playing, without
  sound, because it only gets that picture by playing.
- Leaving the animation or composition tab now silences their video
  preview at once instead of after 2 to 4 seconds.
- Free videos (Video clock free) keep the old behaviour. A sprite
  destroyed by a behavior is still heard for 2 to 4 seconds.

Tests:
- Unit video-library: 10 green (1 new). Without the new timer the
  new test is red.
- e2e 68: ALL PASSED, 11 checks (68g new). An item of 0.5 s with a
  2 s video plays audibly while it is there, and the player is
  paused and muted in every frame from about 200 ms after it ends.
- Gate on melektaus-atx: green, 305 files, 4737 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, 2060 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2061 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