test(render): add WebM regression test and getEncoderPreset unit tests

- New regression test `webm-transparency`: minimal transparent composition
  rendered to WebM, validates VP9 codec and visual quality at 100
  checkpoints against golden baseline
- Extend regression harness: `renderConfig.format` field ("mp4" | "webm"),
  format-aware output paths and snapshot filenames
- Fix `extractMonoPcm16` to gracefully handle videos without audio
  streams (WebM without audio was throwing instead of returning empty)
- Unit tests for `getEncoderPreset()`: VP9/yuva420p for WebM,
  h264/yuv420p for MP4, preset mapping, quality preservation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/producer/tests/webm-transparency/meta.json

@@ -0,0 +1,14 @@
+{
+  "name": "webm-transparency",
+  "description": "WebM VP9 render with transparent background (alpha channel). Validates the full WebM pipeline: PNG capture, VP9 encoding with yuva420p, and alpha_mode=1 metadata.",
+  "tags": ["webm", "transparency", "fast"],
+  "minPsnr": 30,
+  "maxFrameFailures": 2,
+  "minAudioCorrelation": 0.0,
+  "maxAudioLagWindows": 120,
+  "renderConfig": {
+    "fps": 30,
+    "format": "webm",
+    "workers": 1
+  }
+}