fix(gnss): date replayed snapshots by the proxy-reported age, not cross-clock fetchedAt

The layer compared the server's fetchedAt with the browser's clock, so a
skewed client clock could shift or void the 30-minute window. The proxy now
returns ageMs (its own now minus fetchedAt, at serve time, on fresh, cached
and stale answers); the source refuses a snapshot without a finite,
non-negative ageMs, and the layer dates rows as its now() - ageMs. fetchedAt
stays in the payload for display.

Tests: proxy cached/stale answers report ageMs on the server clock; the
source rejects missing/invalid/negative ageMs; the layer replay test drives
ageMs. qa-gnss stubs send ageMs.

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

src/layers/gnss/source.test.mjs

@@ -10,6 +10,7 @@ test('the source asks the same-origin proxy for the view anchor', async () => {
       assert.ok(options.signal instanceof AbortSignal);
       return Response.json({
         fetchedAt: 5,
+        ageMs: 0,
         classifier: 'gev-nic-nacp-v1',
         rows: [{ hex: 'abc123' }],
       });
@@ -22,6 +23,7 @@ test('the source asks the same-origin proxy for the view anchor', async () => {
   assert.deepEqual(calls, ['/api/gnss-integrity?lat=50.06&lon=19.94']);
   assert.deepEqual(snapshot, {
     rows: [{ hex: 'abc123' }],
+    ageMs: 0,
     fetchedAt: 5,
     stale: false,
     classifier: 'gev-nic-nacp-v1',
@@ -31,16 +33,17 @@ test('the source asks the same-origin proxy for the view anchor', async () => {
 test('the source reports stale proxy data and rejects bad input or responses', async () => {
   const reply = (body, init) => async () => Response.json(body, init);
   const stale = await createAdsbGnssSource({
-    fetchImpl: reply({ rows: [], fetchedAt: 7, stale: true }),
+    fetchImpl: reply({ rows: [], fetchedAt: 7, ageMs: 90_000, stale: true }),
   }).getSnapshot({ latitude: 1, longitude: 2 });
   assert.equal(stale.stale, true);
-  assert.equal(stale.fetchedAt, 7, 'a stale reply keeps its observation time');
+  assert.equal(stale.ageMs, 90_000, 'a stale reply keeps its observation age');
+  assert.equal(stale.fetchedAt, 7);
   assert.equal(stale.classifier, null);
-  // Without an observation time the rows could only be dated as new.
-  for (const fetchedAt of [undefined, null, '7'])
+  // Without an observation age the rows could only be dated as new.
+  for (const ageMs of [undefined, null, '7', -1, Number.NaN])
     await assert.rejects(
       createAdsbGnssSource({
-        fetchImpl: reply({ rows: [], fetchedAt }),
+        fetchImpl: reply({ rows: [], fetchedAt: 7, ageMs }),
       }).getSnapshot({ latitude: 1, longitude: 2 }),
       /no observation time/,
     );