perf(terrain): evict from the front of the point cache instead of sorting it

Once the cache was full, every write sorted all 50,000 entries to find the
oldest. A Map keeps insertion order, so keep the map in fetch-time order
and delete from the front: the resolver re-inserts each point it refetches
(an overwrite would leave it in its old place), and the disk load sorts
once, since a file written before this change, or a migrated raw-batch
cache, is not in age order.

Suggested by asturksever on #975.

src/data/terrainHeightsProxy.js

@@ -233,6 +233,9 @@ export async function resolveTerrainHeightRequest({
       for (let i = 0; i < missing.length; i += 1) {
         const result = fetched[i];
         if (validTerrainResult(result)) {
+          // Re-insert rather than overwrite: a Map keeps an existing key in
+          // its old position, and the proxy evicts from the front.
+          cache.delete(missing[i].key);
           cache.set(missing[i].key, { at: fetchedAt, result });
           cacheChanged = true;
         } else if (absentTerrainDatum(result)) {