perf(terrain): one shared, cached and budgeted terrain height reader for Transit, Rail, Bike Share and local data

globe.getHeight is a ray cast against the drawn tile's mesh and
scene.sampleHeight a GPU depth render; several layers asked for them
uncached and unbudgeted. The cache written for Transit moves to
src/services/terrainSurface.js and is handed to every layer through the
ground service, so layers share one cache and one read budget.

- Rail reads each train on a ~4 m grid instead of every frame.
- Local data layers read terrain floors through it and stop depth samples
  at 8 ms a frame.
- Bike Share draws stations at once and finds their heights afterwards in
  4 ms steps.

Co-Authored-By: Claude Fable 5.2 <[email protected]>

src/services/groundFloor.js

@@ -1,3 +1,4 @@
+import { createTerrainSurface } from './terrainSurface.js';
 /** Construct an instance-owned groundFloor service with explicit dependencies. */
 export function createGroundFloor({ terrain, signal }) {
   const { cachedRealEllipsoidalGround, resolveEllipsoidalGround } = terrain;
@@ -698,6 +699,8 @@ export function createGroundFloor({ terrain, signal }) {
     { once: true },
   );
   return {
+    // The terrain globe's loaded surface, read once per point for every layer.
+    terrainSurface: createTerrainSurface(),
     GROUND_FLOOR_LIFT_M,
     MESH_FLOOR_BELOW_PRIOR_M,
     MESH_FLOOR_ABOVE_PRIOR_M,