fix(camera): let a camera above the zoom ceiling zoom in without a jump

Cesium clamps any zoom that starts above maximumZoomDistance to that
limit, in either direction, so a share link or flight that left the
camera above 40,000 km jumped to the ceiling on the first wheel tick.
While the camera is above the ceiling, the guard now uses its current
height as the limit and lowers it as the camera descends.

docs/CURRENT-STATE.md

@@ -45,8 +45,10 @@ its distance-aware zoom, collision handling and minimum/maximum distance limits.
 Fine-pointer desktop mouse and trackpad sensitivity is unchanged.
 
 User zoom is capped at 40,000 km (or a stricter host limit). Explicit camera
-flight, share-link and story destinations can go farther; Cesium can still use
-the limit when it computes an automatic bounding-sphere framing distance.
+flight, share-link and story destinations can go farther; from there, user zoom
+can't climb any higher and zooms in gradually from the current height, with no
+jump to the cap. Cesium can still use the limit when it computes an automatic
+bounding-sphere framing distance.
 Above 300 km, outward movement from the controller's configured zoom bindings
 (wheel, pinch distance and unmodified right-drag by default) pitches the camera
 down towards 8° below the Earth's limb. Large steps reach the limb immediately;