Sample 68d's video wait from before the click on Play (#417)

CI task 669 failed in e2e 68d with no loading frame seen. The check
began to sample after play(), and play() settles after the start. A
video that loads quickly was ready by then, so the wait it checks had
already passed. The composition itself had waited: its clock read 0
when the player was ready.

- 68d starts its sampler before the click on Play and counts only
  frames in play mode.
- The sampler stops after 20 s, not after a number of frames. Before
  the run the page draws far faster than 60 frames a second.

Tests:
- e2e 66, 67, 68 on melektaus-atx: ALL PASSED, 43 checks, 68d saw 6
  loading frames at 0 and 0.32 s after

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/e2e/68-composition.mjs

@@ -185,23 +185,30 @@ export async function composition(t) {
     c.objects = [{ ...structuredClone(c.objects[0]), id: 'cp68-v', assetId: 'cp68-screen', name: 'Video 68', overrides: {} }];
     c.items = [{ id: 'cp68-v', track: 'cp68-front', start: 0, length: 120, offset: 0 }];
     c.clips = [];`);
-  await play();
   // The video time the composition writes into its item every frame.
   const clock = `__gm.world()?.entities.find((e) => e.objectId === '${id('v')}')?.props.videoTime ?? null`;
-  const wait = JSON.parse(
+  // Sampled from before the click on Play: play() settles after the start, and a video that loads
-    await evaluate(`new Promise((done) => {
+  // quickly is ready by then, with no wait left to see (CI task 669).
-      const loading = [];
+  await evaluate(`{
+    const loading = [];
+    const until = performance.now() + 20000;
+    window.cp68Wait = new Promise((done) => {
       const step = () => {
-        const time = ${clock};
+        const playing = __gm.state.mode === 'play';
-        const player = __gm.gameVideo('${id('v')}');
+        const time = playing ? ${clock} : null;
+        const player = playing ? __gm.gameVideo('${id('v')}') : null;
         if (player?.ready) return done(JSON.stringify({ loading, ready: time }));
         if (player) loading.push(time);
-        if (!__gm.world() || loading.length > 300) return done(JSON.stringify({ loading, ready: null }));
+        if (performance.now() > until) return done(JSON.stringify({ loading, ready: null }));
         requestAnimationFrame(step);
       };
       step();
-    })`),
+    });
-  );
+    // Not the promise as the block's value: evaluate would wait for it, and Play would come too late.
+    null;
+  }`);
+  await play();
+  const wait = JSON.parse(await evaluate('window.cp68Wait'));
   await sleep(500);
   const after = await json(clock);
   check(