fix(terrain): bound the client terrain-height cache with LRU eviction

The in-memory cache in the terrain-height resolver had no eviction, so a
session retained every coordinate it ever resolved for the lifetime of the
page. The 5-decimal rounding documented in the file header makes repeated
visits to one place share a key, but it does not bound how many distinct
places a session visits.

Cap it at 20 000 entries, overridable per instance so the bound is testable
without minting twenty thousand coordinates. Eviction is least-recently-used
rather than insertion-ordered because revisits here are spatial: the cell the
camera is parked on must not be dropped for having been resolved early.

The bound sits far above a city-scale session at the ~111 m floor grid, so
normal use never evicts and issues no additional proxy requests. Batch results
are assembled from what the call resolved, so eviction during a large batch
cannot report a just-resolved point as unresolved.

scripts/format-scope.json

@@ -609,6 +609,7 @@
   "src/data/surfaceServices.js",
   "src/annotations/resolver.js",
   "src/services/terrainHeights.js",
+  "src/services/terrainHeights.test.mjs",
   "src/services/groundFloor.js",
   "src/services/meshFloorSampler.js",
   "src/services/groundSnap.js",