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.

CHANGELOG.md

@@ -13,6 +13,8 @@
 
 - Preserve Nepal shot camera and map ownership through the public layer lifecycle; passive restoration does not start standalone playback.
 
+- Bound the client terrain-height cache at 20 000 entries with least-recently-used eviction, so a long session no longer retains every coordinate it ever resolved. Consumer reads promote their entry and a batch still reports every point it resolved.
+
 - Separate transit snapshot/history acquisition from the layer and expose its bounded request service independently of Vite. Preserve feed selection, playback, cache policy and compatibility exports.
 
 - Widen opaque sensor halos to 3 display pixels with a 30 px fleet core while retaining at least 60% opaque core coverage, adding contrast margin through thermal blur and bloom on bright roofs.