fix(perimeters): report a stale WFIGS snapshot as stale, and stop serving it after a day

When WFIGS was unreachable the proxy served its last snapshot, marked stale
(X-Data-Stale, body.stale), with no age limit, and the client source kept
only the rows. The layer then read healthy with a fresh update time and
get_fire_perimeters reported the perimeters as current; three days into an
outage it still said "1 mapped wildfire perimeter in the requested box."
The source now offers getSnapshotDetail ({rows, stale, fetchedAt}); the
layer reports stale with the snapshot's fetch time, the tool adds stale,
retrieved_at and "(data may be stale)", and the proxy serves a stale
snapshot for at most 24 hours, as the GNSS proxy caps its own.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/perimeters/source.test.mjs

@@ -115,3 +115,60 @@ test('response-body completion honors cancellation without replacing records', a
     name: 'AbortError',
   });
 });
+
+test('a failed refresh serves the last snapshot marked stale for a day, then refuses', async () => {
+  let clock = 1_000_000_000;
+  let up = true;
+  const plugin = firePerimetersProxy({
+    now: () => clock,
+    fetchImpl: async () => {
+      if (!up) throw new Error('WFIGS unreachable');
+      return Response.json(validPayload);
+    },
+  });
+  let handler;
+  plugin.configureServer({
+    middlewares: { use: (_path, callback) => (handler = callback) },
+  });
+  const ask = async () => {
+    const reply = {};
+    await handler(
+      { url: '/', method: 'GET', socket: { remoteAddress: '127.0.0.1' } },
+      {
+        writeHead(status, headers) {
+          reply.status = status;
+          reply.staleHeader = headers?.['X-Data-Stale'] === 'true';
+        },
+        end(body) {
+          reply.body = JSON.parse(body);
+        },
+      },
+    );
+    return reply;
+  };
+  const fresh = await ask();
+  assert.equal(fresh.status, 200);
+  assert.equal(fresh.body.stale, undefined);
+  up = false;
+  clock += 23 * 60 * 60 * 1000;
+  const stale = await ask();
+  assert.equal(stale.status, 200);
+  assert.equal(stale.staleHeader, true);
+  assert.equal(stale.body.stale, true);
+  assert.equal(stale.body.fetchedAt, fresh.body.fetchedAt);
+  clock += 2 * 60 * 60 * 1000;
+  assert.equal((await ask()).status, 502, 'a day-old snapshot is not served');
+});
+
+test('the snapshot detail carries the proxy staleness and fetch time', async () => {
+  const fetchedAt = Date.UTC(2026, 9, 25, 12);
+  const source = createWfigsPerimeterSource({
+    fetchImpl: async () => Response.json({ rows: [], stale: true, fetchedAt }),
+  });
+  assert.deepEqual(await source.getSnapshotDetail(), {
+    rows: [],
+    stale: true,
+    fetchedAt,
+  });
+  assert.deepEqual(await source.getSnapshot(), []);
+});