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/SoundLibrary.ts

@@ -614,6 +614,25 @@ export class SoundLibrary implements AudioSink {
     return readMeter((this.guardMeter ??= this.tapMeter(guard.limiter)));
   }
 
+  /**
+   * A node that hears what leaves the output, limiter and all (#405): a recording of the game view
+   * takes its sound there, in pieces of both sides (`record-tap`). Null without a live output.
+   */
+  async outputTap(): Promise<{ node: AudioWorkletNode; rate: number; release(): void } | null> {
+    const context = this.audio();
+    if (!(context instanceof AudioContext) || !(await this.fxWorklet())) return null;
+    const guard = this.guard;
+    if (!guard) return null;
+    const node = new AudioWorkletNode(context, 'record-tap', {
+      numberOfOutputs: 0,
+      channelCount: 2,
+      channelCountMode: 'explicit',
+    });
+    const source = guard.limiter ?? guard.input;
+    source.connect(node);
+    return { node, rate: context.sampleRate, release: () => source.disconnect(node) };
+  }
+
   /** Lets every level meter go. */
   unmeter(): void {
     for (const bus of this.buses.values()) this.dropMeter(bus);