refactor(street-level): drop FOLLOW and Google 3D ground casting

FOLLOW is gone: no setFollow, follow state, map-stack availability or
handoff listener. Opening a photo still frames it with one flight through
the application's deferred navigation (begin/reassert), so a slow photo
never overrides newer navigation and tracking is released as before.

Ground casting is gone: groundCast.js, meshSampler.js, the surface mode,
the draped-to-cast swap and remesh. Coverage lines are always draped
GroundPolylinePrimitives (ClassificationType.BOTH); cones and the marker
clamp to the ground and keep the horizon cull. The terrain/meshFloor/ground
services and meshFloor.visibleTilesetLoaded are no longer needed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/services/meshFloorSampler.js

@@ -195,9 +195,5 @@ export function createMeshFloorSampler({
     () => eventTarget?.removeEventListener('gev:map-stack-changed', onStack),
     { once: true },
   );
-  return {
-    sampleMeshFloorCells,
-    cachedGroundFloor,
-    visibleTilesetLoaded: _visibleTilesetLoaded,
-  };
+  return { sampleMeshFloorCells, cachedGroundFloor };
 }