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/core/graph/nodes/action/recordingNodes.ts

@@ -0,0 +1,66 @@
+import { RECORDING_FPS, RECORDING_RESOLUTIONS, recordingSettingsOf } from '../../../recording/recording';
+import type { NodeDef, PortDef } from '../../NodeRegistry';
+import { asString } from '../../values';
+import { exec } from '../shared';
+
+const choicePort = (
+  key: string,
+  label: string,
+  options: readonly { value: string; label: string }[],
+): PortDef => ({
+  key,
+  label: key,
+  type: 'string',
+  default: options[0]!.value,
+  field: { key, label, kind: 'enum', default: options[0]!.value, options: [...options] },
+});
+
+export const StartRecording: NodeDef = {
+  type: 'StartRecording',
+  label: 'Start recording',
+  category: 'action',
+  description:
+    'Records the game view as a video, with what the master plays: only the picture of the camera, ' +
+    'none of what lies over it. "name" is what the recording is saved as; left empty, it is named ' +
+    'after the moment it starts. "resolution" Game view records at the size the picture has on ' +
+    'screen when it starts, 720p and 1080p at that height with the width of the camera. The recording ' +
+    'goes on through Load scene and Restart game, holds while the editor pauses the run, and ends with ' +
+    'Stop recording or with the run. In the editor it becomes a video asset of the project, once the ' +
+    'server has converted it; a released game downloads it as an MKV file. A second start while one ' +
+    'records is ignored and says so in the log.',
+  inputs: [
+    exec(),
+    {
+      key: 'name',
+      label: 'name',
+      type: 'string',
+      field: { key: 'name', label: 'Name', kind: 'text', default: '' },
+    },
+    choicePort('resolution', 'Resolution', RECORDING_RESOLUTIONS),
+    choicePort('fps', 'FPS', RECORDING_FPS),
+  ],
+  outputs: [exec()],
+  params: [],
+  run: (ctx, inputs) => {
+    ctx.startRecording(
+      recordingSettingsOf(asString(inputs.name), asString(inputs.resolution), asString(inputs.fps)),
+    );
+    return { next: 'exec' };
+  },
+};
+
+export const StopRecording: NodeDef = {
+  type: 'StopRecording',
+  label: 'Stop recording',
+  category: 'action',
+  description:
+    'Ends the recording Start recording began and saves it. Without one running it does nothing but ' +
+    'say so in the log.',
+  inputs: [exec()],
+  outputs: [exec()],
+  params: [],
+  run: (ctx) => {
+    ctx.stopRecording();
+    return { next: 'exec' };
+  },
+};

src/core/graph/nodes/index.ts

@@ -33,6 +33,7 @@ import { StopSong } from './action/StopSong';
 import { SetZ } from './action/SetZ';
 import { Spawn } from './action/Spawn';
 import { StopGame } from './action/StopGame';
+import { StartRecording, StopRecording } from './action/recordingNodes';
 import { StopSound } from './action/StopSound';
 import { CellToWorld, SetTile, TileAt, TileLiteral, WorldToCell } from './action/tileNodes';
 import { Abs } from './data/Abs';
@@ -323,6 +324,8 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   Log,
   StopGame,
   RestartGame,
+  StartRecording,
+  StopRecording,
   PauseGame,
   ResumeGame,
   IsGamePaused,