fix(engine): use domcontentloaded to avoid video preload navigation timeout (#1239)

Page.goto with waitUntil:'load' blocks until all video sources finish
preloading metadata. With 6+ video sources (8–21s each), this reliably
exceeds the 60s navigation timeout, causing renders to hang at
'Initializing calibration session...' on M4 and other Apple Silicon macs.

Switch to waitUntil:'domcontentloaded' so navigation resolves as soon as
the DOM is ready, without gating on video media loading.

Fixes #1231

packages/producer/src/parity-harness.ts

@@ -284,8 +284,10 @@ async function captureParitySide(
 ): Promise<{ buffer: Buffer; styles: Record<string, unknown> }> {
   const page = await browser.newPage();
   try {
+    // Use domcontentloaded to avoid blocking on video media preloading, which
+    // can exceed the navigation timeout for compositions with many video sources.
     await page.goto(url, {
-      waitUntil: "load",
+      waitUntil: "domcontentloaded",
       timeout: 60_000,
     });
     await waitForParityReady(page);