Keep a recording going behind another browser tab

Chrome stops calling requestAnimationFrame for a hidden tab, and Pixi's
ticker hangs on it. The run, the drawing and the recording all stood
still as soon as the user switched to another tab. A plain run keeps
standing still there, so a forgotten game does not play on unheard.

- backgroundBeat.ts ticks the editor from a worker at the recording's
  fps while a recording goes and the tab is hidden, or the page has
  not drawn a frame of its own for 250 ms. Once the page draws again,
  the beat keeps out of the way.
- The worker looks every 4 ms whether a tick is due. A 16 ms interval
  came in bursts behind a busy page, and the recording threw away the
  second tick of each burst.
- backgroundTimers.ts and timerWorker.ts give the short waits of a
  recording timers in a worker. Chrome slows the page's own timers to
  once a second in a hidden tab. This covers the waits while saving
  and the wait for a video's picture after a seek.
- Behind a tab Chrome does not call requestVideoFrameCallback, but the
  picture is there at seeked. The wait there is 10 ms instead of
  150 ms while the tab is hidden.
- The beat runs for Start recording, the exported game and Start
  recording... in the main menu.

Tests:
- tsc, eslint, Prettier clean
- tests/viewport: 374 green, 3 new in background-beat.test.ts
- e2e 68k (3 checks) on melektaus-atx with the tab really hidden:
  not in real time all 120 frames, colour changes every 30 frames,
  the blip 8 ms from green; in real time 97 of 120 frames (73 with
  the tab in front)
- Counter-check without the beat: all three 68k checks red, 3 frames
  in 60 s not in real time, 7 frames in real time
- Gate on melektaus-atx: green, 311 files, 4821 tests

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

src/viewport/timerWorker.ts

@@ -0,0 +1,30 @@
+/**
+ * The timers of `backgroundTimers.ts`: a worker's keep their pace in a hidden tab, where the page's
+ * own are held to once a second. `{ id, ms, repeat }` starts one, `{ id, ms: 0 }` ends it; each time
+ * it fires, its id comes back.
+ */
+const timers = new Map<number, ReturnType<typeof setTimeout>>();
+
+self.onmessage = (event: MessageEvent<{ id: number; ms: number; repeat: boolean }>) => {
+  const { id, ms, repeat } = event.data;
+  const running = timers.get(id);
+  if (running !== undefined) {
+    clearTimeout(running);
+    clearInterval(running);
+    timers.delete(id);
+  }
+  if (ms <= 0) return;
+  if (repeat)
+    timers.set(
+      id,
+      setInterval(() => self.postMessage(id), ms),
+    );
+  else
+    timers.set(
+      id,
+      setTimeout(() => {
+        timers.delete(id);
+        self.postMessage(id);
+      }, ms),
+    );
+};