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-harness-baseline.json

@@ -1,8 +1,8 @@
 {
   "metrics": {
     "ownLaunch": 98,
-    "fixedSleeps": 117,
-    "loadingScreenReadiness": 9,
+    "fixedSleeps": 85,
+    "loadingScreenReadiness": 1,
     "cesiumOnlyReads": 75,
     "ownWaitHelpers": 19
   }