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/runtime/systems/behaviorContext.ts

@@ -42,6 +42,7 @@ import type { World } from '../World';
 import type { AudioSink, NoteTiming } from './Audio';
 import type { Instance } from './behaviorPulses';
 import type { Input } from './Input';
+import type { RecordingSink } from '../../core/recording/recording';
 import { bodyOf, type CollisionSource, layerHolderOf, NOT_BLOCKED } from './solids';
 
 export interface TimeScaled {
@@ -55,6 +56,8 @@ export interface ContextSources {
   collisions: CollisionSource | null;
   audio: AudioSink;
   clock: TimeScaled | null;
+  /** Where a recording of the game view goes (#405); only a played game has one. */
+  recorder: RecordingSink | null;
   shared: SharedState;
   /** A graph moved one of this instance's machines; its events follow between exec flows. */
   stateChanged(machineId: string, from: string, to: string): void;
@@ -265,9 +268,20 @@ export function behaviorContext(
     setPaused: (paused) => {
       world.paused = paused;
     },
+    startRecording: (settings) => tell(world, run.recorder?.start(settings) ?? NO_RECORDER),
+    stopRecording: () => tell(world, run.recorder?.stop() ?? NO_RECORDER),
   };
 }
 
+const NO_RECORDER = 'Recording works only while the game is played, not in the editor run';
+
+/** A line of the host for the log, as the `Log` node writes one; nothing for an empty one. */
+function tell(world: World, line: string): void {
+  if (!line) return;
+  world.log.push(line);
+  console.warn('[recording]', line);
+}
+
 /**
  * What the graph asked for, as the sink understands it: the asset's own volume scaled by the
  * node's, and its loop flag unless the node overrules it either way. The asset is what a sound