test(terrain): pin which point the cache evicts after a refetch

A stale point that is refetched becomes the newest entry, and the cache
file's order is not its age order. The ceiling must still evict the
oldest fetch. Passes on the current sort-based eviction.

src/terrainHeightsBounds.test.mjs

@@ -165,6 +165,44 @@ test('the point cache stops growing, and an evicted point is simply refetched',
   assert.equal(asked[4], point(1));
 });
 
+test('a refetched point counts as new when the cache evicts', async (t) => {
+  const root = await mkdtemp(path.join(tmpdir(), 'terrain-evict-order-'));
+  t.after(() => rm(root, { recursive: true, force: true }));
+  const cachePath = path.join(root, 'cache.json');
+  const day = 24 * 3600_000;
+  const now = Date.now();
+  const point = (i) => `-97.7${i}000,30.26000`;
+  const entry = (at) => ({ at, result: { ellipsoid: 150 } });
+  // Written newest first: the file's order is not the age order. Point 1 is
+  // past the 30-day TTL, so the next request for it refetches.
+  await writeFile(
+    cachePath,
+    JSON.stringify({
+      version: 2,
+      points: {
+        [point(3)]: entry(now - 3600_000),
+        [point(2)]: entry(now - day),
+        [point(1)]: entry(now - 31 * day),
+      },
+    }),
+  );
+  const asked = stubTerrainUpstream(t);
+  const route = install(
+    terrainHeightsProxy({ cacheMaxEntries: 3, cachePath }),
+  ).get('/api/terrain/heights');
+
+  await request(route, { url: `/?points=${point(1)}` });
+  assert.deepEqual(asked, [point(1)], 'the stale point is refetched');
+  await request(route, { url: `/?points=${point(4)}` });
+  assert.equal(asked.length, 2);
+
+  // Point 2 is now the oldest fetch; point 1 was just refreshed.
+  await request(route, { url: `/?points=${point(1)};${point(3)}` });
+  assert.equal(asked.length, 2, 'the refreshed and the recent point stay');
+  await request(route, { url: `/?points=${point(2)}` });
+  assert.deepEqual(asked.slice(2), [point(2)], 'the oldest fetch was evicted');
+});
+
 test('an over-large cache file is not read back into memory', async (t) => {
   const asked = stubTerrainUpstream(t);
   const point = '-97.74000,30.26000';