Read a driven video's speed on the frame clock so 1x stays 1x (#404)

A driven video measures how fast its wearer's time runs and sets its
playbackRate from that. The time moves on with the ticker, but the
reading took its clock from performance.now() while drawing. A frame
that had more to do before the draw shifted that clock against the
time, and a 1x animation read as up to 1.09. Outside 0.9 to 1.1 the
video goes mute, so the sound could drop out for a moment. In CI this
made e2e 66k red now and then.

- VideoLibrary reads the speed with the timestamp of the frame
  (document.timeline.currentTime), the same clock the time runs on.
  It falls back to performance.now() where there is no document.
- debugInfo() reports the measured speed of a driven player.
- e2e 66k checks the measured speed and the correction of the rate
  apart. The rate alone may differ from 1 by up to 10 percent on
  purpose, so checking it at +-0.1 was too strict.

Tests:
- Unit video-library: 8 tests green, 1 new. With performance.now()
  back in place the new test is red and reads 1.089.
- e2e 66 on melektaus-atx: all passed, 27 checks, speed 1.000 in
  all 5 readings.
- Gate on melektaus-atx: CHECK EXIT 0, 297 files, 4562 tests.

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

tests/viewport/video-library.test.ts

@@ -107,7 +107,10 @@ describe('VideoLibrary driven players (#404)', () => {
     vi.stubGlobal('document', { hidden: false, addEventListener: () => {}, createElement: fakeVideo });
   });
 
-  afterEach(() => vi.unstubAllGlobals());
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.restoreAllMocks();
+  });
 
   const REF = { file: KEY, smooth: true, rect: null, sourceWidth: 320, sourceHeight: 180, slice: null };
 
@@ -128,6 +131,30 @@ describe('VideoLibrary driven players (#404)', () => {
     ]);
   });
 
+  it('reads the speed by the time of the frame, not by when the frame got round to it', async () => {
+    const frame = { at: 0 };
+    vi.stubGlobal('document', {
+      hidden: false,
+      addEventListener: () => {},
+      createElement: fakeVideo,
+      timeline: {
+        get currentTime() {
+          return frame.at;
+        },
+      },
+    });
+    // The first frames are drawn 45 ms late, as on a busy page; the time runs at 1x by the frames.
+    let late = 45;
+    vi.spyOn(performance, 'now').mockImplementation(() => frame.at + late);
+    const videos = await freshLibrary();
+    for (let i = 0; i <= 36; i++) {
+      frame.at = (i * 1000) / 60;
+      if (i === 7) late = 0;
+      videos.wear(REF, { videoClock: 'driven', videoTime: i / 60 }, 'view1:a', false);
+    }
+    expect(videos.debugInfo()[0]!.speed).toBeCloseTo(1, 6);
+  });
+
   it('tells a view whether one of its driven players still waits for its first picture', async () => {
     const videos = await freshLibrary();
     videos.get(KEY);