Fix the trail and decoded-weather probes on both engines

Both probes called waitForEngine without the engine kind, which can never
match and timed out after 120 s; waitForEngine now rejects a missing kind.
The trail probe passed no result vectors to the geo helpers, and drew its
trail below the ground, where only the dimmed occluded pass showed it; it
now draws halfway down to the ground and both engines count the same
pixels.

scripts/qa-strict-harness.mjs

@@ -747,6 +747,8 @@ export async function pickEngine(
 
 /** Wait until `kind` runs, its picker is idle and its view is restored. */
 export async function waitForEngine(page, kind, { timeout = 120000 } = {}) {
+  if (kind !== 'cesium' && kind !== 'navara')
+    throw new TypeError(`waitForEngine needs the engine kind, got ${kind}`);
   const started = Date.now();
   await page.waitForFunction(
     (expected) =>

scripts/qa-trail-freeze.mjs

@@ -20,24 +20,40 @@ try {
   await page.setViewport({ width: 960, height: 640 });
   await blockPaidQaMaps(page);
   await page.goto(url, { waitUntil: 'domcontentloaded' });
-  await waitForEngine(page);
+  await waitForEngine(
+    page,
+    new URL(url).searchParams.get('engine') ?? 'navara',
+  );
   const result = await evaluateWithGeo(page, async (geo) => {
     const { createTrail, CHUNK_POINTS } =
       await import('/src/data/trailRenderer.js');
     const { engine } = window.__godsEyeView;
     engine.camera.cancelFlight();
     const camera = engine.camera.pose();
+    const offset = (direction, metres) =>
+      geo.cartesian3.multiplyByScalar(
+        direction,
+        metres,
+        geo.cartesian3.create(),
+      );
+    // Halfway down to the ground along the view, so the trail is drawn in
+    // front of the surface on both engines rather than through it.
+    const below = geo.cartographic.fromCartesian(camera.position);
+    const clearance = below.height - (engine.surface.terrainHeight(below) ?? 0);
+    const distance = Math.min(
+      3000,
+      (0.5 * clearance) / Math.max(Math.sin(-camera.pitch), 0.05),
+    );
     const ahead = geo.cartesian3.add(
       camera.position,
-      geo.cartesian3.multiplyByScalar(camera.direction, 3000),
+      offset(camera.direction, distance),
+      geo.cartesian3.create(),
     );
     const points = Array.from({ length: CHUNK_POINTS + 1 }, (_, index) =>
       geo.cartesian3.add(
         ahead,
-        geo.cartesian3.multiplyByScalar(
+        offset(camera.right, distance * (-0.4 + (0.8 * index) / CHUNK_POINTS)),
-          camera.right,
+        geo.cartesian3.create(),
-          -1200 + (2400 * index) / CHUNK_POINTS,
-        ),
       ),
     );
     const gl =

scripts/qa-weather-decoded-tiles.mjs

@@ -17,7 +17,7 @@ try {
     const url = new URL(base);
     url.search = `?engine=${kind}&welcome=0`;
     await page.goto(url.href, { waitUntil: 'domcontentloaded' });
-    await waitForEngine(page);
+    await waitForEngine(page, kind);
     await evaluateWithGeo(page, (geo) => {
       window.__godsEyeView.engine.camera.setView({
         destination: geo.cartesian3.fromDegrees(-79.38, 43.65, 500000),