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]>

docs/INFRASTRUCTURE-LAYERS.md

@@ -34,6 +34,7 @@ const layers = createInfrastructureLayers({
 | `clearSelectedEntityContextForLayer` | `(layerId)`; clear selection only if owned by this layer |
 | `removeEntityContextsForLayer` | `(layerId)`; remove records owned by the layer on destruction or failed setup |
 | `governorRequestRender` | `(reason)`; request a frame through the existing render scheduler |
+| `terrainSurface` | Optional. The application's shared terrain surface (`groundFloor.terrainSurface`), so its layers read each terrain height once; without it each layer keeps its own |
 
 Each returned layer supports `init(viewer)`, `enable(viewer)`, `disable(viewer)`,
 `update(viewer)`, `destroy(viewer)`, `getStats()`, `getLodDiagnostics()` and