fix(qa): don't pass the macOS-only Metal ANGLE backend on other platforms

Six QA harnesses launched Chrome with `--use-angle=metal` unconditionally
(or defaulted to it). Metal is a macOS-only ANGLE backend: on Windows and
Linux it makes WebGL initialization fail outright, so CesiumWidget never
constructs, the app never boots, and the harness dies at its
`waitForFunction` boot wait — before running a single assertion:

    Error constructing CesiumWidget.
    RuntimeError: The browser supports WebGL, but initialization failed.
    TimeoutError: Waiting failed: 60000ms exceeded

Affected: qa-map-source-tray, qa-cockpit-utility, qa-view-target-prewarm,
qa-firstrun (unconditional), qa-cockpit-plates (GPU branch taken off-Mac
too) and qa-floor-hold (`--angle` defaulted to metal everywhere).

Fixed by adopting the platform check qa-flyroute-cinema.mjs already uses:
real GPU via Metal on darwin, the portable swiftshader rasterizer
elsewhere — which is what the other ~20 QA scripts already pass. No
behavior change on macOS.

Verified on Windows 11: `npm run qa:map-source-tray` went from 0
assertions (boot timeout) to all 20 checks running.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01C3eGYgDYBo2BCUeDGwGvrp

scripts/qa-map-source-tray.mjs

@@ -29,7 +29,15 @@ fs.mkdirSync(shotsDir, { recursive: true });
 const browser = await puppeteer.launch({
   headless: headful ? false : 'new',
   executablePath,
-  args: ['--use-angle=metal', '--enable-gpu', '--no-sandbox'],
+  // Metal is a macOS-only ANGLE backend: passing it on Windows or Linux makes
+  // WebGL initialization fail outright, so Cesium never constructs and this
+  // harness dies at the boot wait before a single assertion runs.
+  args: [
+    ...(process.platform === 'darwin'
+      ? ['--use-angle=metal', '--enable-gpu']
+      : ['--use-gl=angle', '--use-angle=swiftshader']),
+    '--no-sandbox',
+  ],
 });
 const page = await browser.newPage();
 const failures = [];