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/browserManager.ts

@@ -108,11 +108,7 @@ export async function acquireBrowser(
   } else {
     // Screenshot mode with renderSeek: works on all platforms.
     captureMode = "screenshot";
-    if (headlessShell && !isLinux) {
-      executablePath = headlessShell;
-    } else {
-      executablePath = undefined; // Puppeteer uses its bundled Chromium
-    }
+    executablePath = headlessShell ?? undefined;
   }
 
   const ppt = await getPuppeteer();

packages/engine/src/services/frameCapture.ts

@@ -108,9 +108,10 @@ export async function createCaptureSession(
   await page.setViewport(viewport);
 
   // For PNG capture (used by WebM/transparency), make the page background transparent
-  // so Chrome's screenshot captures alpha channel data.
+  // so Chrome's screenshot captures alpha channel data. Must use the same CDP session
+  // that the screenshot service uses (getCdpSession caches per page).
   if (options.format === "png") {
-    const cdp = await page.createCDPSession();
+    const cdp = await getCdpSession(page);
     await cdp.send("Emulation.setDefaultBackgroundColorOverride", {
       color: { r: 0, g: 0, b: 0, a: 0 },
     });

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");

packages/producer/src/services/renderOrchestrator.ts

@@ -301,6 +301,10 @@ export async function executeRenderJob(
   const perfStages: Record<string, number> = {};
   const perfOutputPath = join(workDir, "perf-summary.json");
   const cfg = job.config.producerConfig ?? resolveConfig();
+  // WebM/transparency requires screenshot mode — beginFrame doesn't support alpha channel
+  if (job.config.format === "webm") {
+    cfg.forceScreenshot = true;
+  }
   const enableChunkedEncode = cfg.enableChunkedEncode;
   const chunkedEncodeSize = cfg.chunkSizeFrames;
   const enableStreamingEncode = cfg.enableStreamingEncode;