Wait for the application, not the clock, in the strict QA set

The probe exposes settled() and the first-frame outcome; scripts/qa/ready.mjs
waits for drawn frames with the camera at rest and for a pick at the click
position, with a deadline and its reason in the error. The strict scenarios
use these instead of fixed pauses and loading-screen polling, and the
hidden-switch scenario checks how the loading screen was lifted instead of
how long it took.

Each switch scenario now records a draw ledger per engine and runs a
contract monitor; both write to the scenario's evidence file only, with an
empty allowlist until browser runs show what differs on purpose.

scripts/qa-weather-engine-switch.mjs

@@ -1,3 +1,4 @@
+import { whenSettled } from './qa/ready.mjs';
 import { fromDegrees } from '../src/geo/cartesian3.js';
 import sharp from 'sharp';
 import assert from 'node:assert/strict';
@@ -182,7 +183,7 @@ await strict.run({}, async (b) => {
       time,
     );
   const sample = async () => {
-    await new Promise((r) => setTimeout(r, 500));
+    await whenSettled(p);
     const crop = await sharp(await p.screenshot())
       .extract({ left: 535, top: 388, width: 12, height: 12 })
       .removeAlpha()