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/layers/bikeshare/policy.js

@@ -50,6 +50,17 @@ export const DEFAULT_CAPACITY = 15;
 
 export const POINT_HEIGHT_OFFSET_M = 2.0;
 
+/**
+ * Milliseconds one step may spend finding station ground heights. A height
+ * the terrain globe cannot give is a depth render of a millisecond or more.
+ */
+
+export const HEIGHT_REFINE_BUDGET_MS = 4;
+
+/** Milliseconds between those steps, about a frame. */
+
+export const HEIGHT_REFINE_INTERVAL_MS = 16;
+
 /** Hard cap on total rendered station points across all cities. */
 
 export const MAX_TOTAL_POINTS = 8000;