fix(render): fix WebM transparency — use screenshot mode + proper CDP setup

- Force screenshot capture mode for WebM (beginFrame doesn't support
  alpha channel in chrome-headless-shell)
- Set Emulation.setDefaultBackgroundColorOverride once during session
  creation (matching experiment-framework's approach)
- Fix acquireBrowser to pass executable path in screenshot mode on
  Linux (was setting undefined, causing puppeteer-core to fail)
- Add Page.captureScreenshot params: fromSurface, captureBeyondViewport,
  optimizeForSpeed (matching experiment-framework)
- Fix regression harness extractMonoPcm16 for videos without audio
- Add getEncoderPreset unit tests
- Add webm-transparency regression test with golden baseline

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

packages/engine/src/services/screenshotService.ts

@@ -104,6 +104,8 @@ export async function pageScreenshotCapture(page: Page, options: CaptureOptions)
   const result = await client.send("Page.captureScreenshot", {
     format,
     quality: format === "jpeg" ? (options.quality ?? 80) : undefined,
+    fromSurface: true,
+    captureBeyondViewport: false,
     optimizeForSpeed: true,
   });
   return Buffer.from(result.data, "base64");