Give QA scripts one launch, readiness and geometry helper

scripts/qa/browser.mjs launches headless Chrome with QA_ANGLE and the
shared Chrome lookup; scripts/qa/ready.mjs adds frame, tile and transition
readiness and bounded polling; scripts/qa/geometry.mjs hands page
evaluations the engine-neutral geometry without dev-only source URLs.
CODE-BOUNDARIES gains the rule for new QA scripts.

scripts/qa/browser.test.mjs

@@ -0,0 +1,39 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import puppeteer from 'puppeteer';
+import { launchBrowser } from './browser.mjs';
+
+test('scenario launch uses the selected ANGLE backend and stays headless', async (t) => {
+  const prior = process.env.QA_ANGLE;
+  t.after(() => {
+    if (prior === undefined) delete process.env.QA_ANGLE;
+    else process.env.QA_ANGLE = prior;
+  });
+  process.env.QA_ANGLE = 'swiftshader';
+  const browser = {};
+  let options;
+  t.mock.method(puppeteer, 'launch', async (value) => {
+    options = value;
+    return browser;
+  });
+  assert.equal(
+    await launchBrowser({
+      headless: false,
+      args: ['--no-sandbox'],
+      defaultViewport: { width: 800, height: 600 },
+      protocolTimeout: 120000,
+    }),
+    browser,
+  );
+  assert.equal(options.headless, true);
+  assert.deepEqual(options.args, [
+    '--use-angle=swiftshader',
+    '--enable-gpu',
+    '--no-sandbox',
+  ]);
+  assert.deepEqual(options.defaultViewport, { width: 800, height: 600 });
+  assert.equal(options.protocolTimeout, 120000);
+  delete process.env.QA_ANGLE;
+  await launchBrowser();
+  assert.equal(options.args[0], '--use-angle=metal');
+});