Record the game view from a behavior into a video asset (#405)

A game can now film itself. A behavior starts and stops a take, and
the editor turns it into a video asset with the run's picture and the
sound the player hears. The exported game hands the file over as a
download, since there is no project to keep it in.

- New action nodes `Start recording` (Name, Resolution, FPS) and
  `Stop recording`. An empty name becomes `Recording <timestamp>`.
- Resolution is `Game view`, `720p` or `1080p`. For the fixed sizes
  the height is set and the width follows the camera.
- The picture is the camera only, without hints, overlays or the
  letterbox. It is drawn into a render texture of the target size,
  so a smaller window still gives full 1080p.
- `PictureReader` reads the texture back through pixel buffers with
  a fence. The editor never waits on the GPU, and a frame with all
  three buffers busy is dropped and counted.
- Nothing of the take reaches the visible canvas. A first version
  drew tiles into it and copied them out, and with the low-latency
  canvas that midway state flickered on screen.
- The sound is tapped behind the master limiter in the FX worklet
  and encoded as Opus.
- H.264 is chosen when the browser has it, VP9 otherwise. A small
  own Matroska writer packs both tracks, no new dependency. It names
  the frame rate and BT.709 so ffmpeg keeps both.
- A second `Start recording` is ignored with a warning in the log.
  A take goes on through `Load scene` and a restart, pauses with the
  editor and is saved when the run ends. It stops at 2 GB.
- The editor uploads the take on the existing video route and makes
  the asset once the run is over. The video tab gets a Download
  button for the converted MP4.
- `downloadBlob` in `src/ui` replaces the song editor's own copy.

Findings:
- `On start` runs before the game screen knows the world. A take
  therefore fixes its size on the first frame that has a camera.
- Without a frame rate in the file, ffmpeg turned 30 fps into 59.94.
- The worker's poster JPEG is off in colour for every upload (red
  255 comes out as 229, BT.601). Not touched here.

Tests:
- Unit recording + matroska: 21 green, plus the node catalog cases.
  The Matroska test reads the file back with its own EBML reader.
- e2e 67 (new) on melektaus-atx: ALL PASSED. 1080p30 from a
  1280x800 window gives a 1920x1080 MP4 with sound and no dropped
  frame. Every 4th pixel is checked, a green quarter catches an
  upside-down picture. A take ended by Esc is saved with the
  timestamp name.
- e2e 60j: the exported game downloads an `.mkv` with EBML magic.
- The flicker is gone on the user's machine without `?vsync`.
- Gate on melektaus-atx: CHECK EXIT 0, 299 files, 4586 tests.

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

src/viewport/fxWorklet.ts

@@ -391,3 +391,52 @@ class ChannelProbe extends AudioWorkletProcessor {
 }
 
 registerProcessor('channel-probe', ChannelProbe);
+
+/** Frames a recording's sound is handed over in: about a tenth of a second. */
+const RECORD_BLOCK = 4096;
+
+/**
+ * Hands what reaches it to the page, both sides, `RECORD_BLOCK` frames at a time (#405): the master
+ * as it leaves the limiter, for a recording of the game view. Told to stop, it sends what it holds
+ * and an empty piece after it, the sign that nothing more comes.
+ */
+class RecordTap extends Stoppable {
+  private readonly sides = [new Float32Array(RECORD_BLOCK), new Float32Array(RECORD_BLOCK)];
+  private filled = 0;
+
+  constructor() {
+    super();
+    this.port.onmessage = () => {
+      this.send();
+      this.port.postMessage([new Float32Array(0), new Float32Array(0)]);
+      this.stop();
+    };
+  }
+
+  process(inputs: Float32Array[][]): boolean {
+    if (this.stopped) return false;
+    const input = inputs[0] ?? [];
+    const left = input[0];
+    const frames = left?.length ?? 128;
+    this.sides.forEach((side, c) => {
+      const data = input[c] ?? left;
+      if (data) side.set(data, this.filled);
+      else side.fill(0, this.filled, this.filled + frames);
+    });
+    this.filled += frames;
+    if (this.filled + frames > RECORD_BLOCK) this.send();
+    return true;
+  }
+
+  private send(): void {
+    if (this.filled === 0) return;
+    const out = this.sides.map((side) => side.slice(0, this.filled));
+    this.filled = 0;
+    this.port.postMessage(
+      out,
+      out.map((side) => side.buffer),
+    );
+  }
+}
+
+registerProcessor('record-tap', RecordTap);