fix(camera): keep the zoom ceiling fixed while tracking a target

While the camera tracks an entity or looks at a point, Cesium measures
zoom distance from the target, not the Earth. Raising the ceiling to the
camera's altitude then lifted it with every outward step, so a tracked
camera could zoom out without limit. The guard now follows the altitude
only for an untracked 3D camera and otherwise keeps its 40,000 km
ceiling.

src/app/zoomOutGuard.js

@@ -111,7 +111,12 @@ export function installZoomOutGuard(
   const followCeiling = () => {
     if (!ownsCeiling || controller.maximumZoomDistance !== maximumZoomDistance)
       return;
-    const height = camera.positionCartographic?.height;
+    // While tracking, Cesium measures zoom from the target, not the Earth, so
+    // the camera's altitude would lift the ceiling with every outward step.
+    const untracked =
+      scene.mode === Cesium.SceneMode.SCENE3D &&
+      Cesium.Matrix4.equals(camera.transform, Cesium.Matrix4.IDENTITY);
+    const height = untracked ? camera.positionCartographic?.height : undefined;
     maximumZoomDistance =
       height > MAXIMUM_ZOOM_DISTANCE_M ? height : MAXIMUM_ZOOM_DISTANCE_M;
     controller.maximumZoomDistance = maximumZoomDistance;

src/app/zoomOutGuard.test.mjs

@@ -741,3 +741,24 @@ test('a share-link view restored above the ceiling is honoured by later frames',
     s.dispose();
   }
 });
+
+test('a tracked camera cannot climb past the ceiling from its target', () => {
+  const s = realControllerScene(1_000_000);
+  try {
+    s.frame();
+    // In look-at mode Cesium measures zoom from the target, not the Earth.
+    s.camera.lookAt(
+      Cesium.Cartesian3.fromDegrees(0, 0, 35_786_000),
+      new Cesium.HeadingPitchRange(0, -0.5, 5_000_000),
+    );
+    for (let i = 0; i < 20; i++) {
+      s.wheel(100);
+      s.frame();
+    }
+    const range = Cesium.Cartesian3.magnitude(s.camera.position);
+    assert.ok(range <= MAXIMUM_ZOOM_DISTANCE_M + 1_000, `${range} m`);
+    assert.equal(s.controller.maximumZoomDistance, MAXIMUM_ZOOM_DISTANCE_M);
+  } finally {
+    s.dispose();
+  }
+});