fix(gnss): age degraded evidence and replayed snapshots on their own clocks

Maintainer review on #855 (mazy1998, 2026-10-06):

- records: presence (seenAt) and degraded evidence (degradedAt) age
  separately, so continuing healthy reports no longer keep an older
  degraded flag alive past 30 minutes; an aircraft returning after the
  window starts fresh. pruneGnssObservations ages the store with no new
  rows.
- Rows are dated by the snapshot's fetchedAt, which the proxy keeps on
  cached and stale answers: a replay never restarts the window, the
  proxy stops serving a stale snapshot older than the window, the source
  refuses an undated one, and cells expire during fetch failures and
  while there is no view anchor.
- Normalization accepts ADS-B versions 1 and 2 only; a missing, invalid
  or negative seen_pos is not a confirmed position from the last minute.
- qa-gnss: keyboard, 390x844 viewport (fresh page: isMobile reloads) and
  replay-age sections; the live cell assertion bins the returned rows
  with the layer's own rules instead of a raw row count.

src/layers/gnss/source.test.mjs

@@ -31,11 +31,19 @@ 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: [], stale: true }),
+    fetchImpl: reply({ rows: [], fetchedAt: 7, stale: true }),
   }).getSnapshot({ latitude: 1, longitude: 2 });
   assert.equal(stale.stale, true);
-  assert.equal(stale.fetchedAt, null);
+  assert.equal(stale.fetchedAt, 7, 'a stale reply keeps its observation time');
   assert.equal(stale.classifier, null);
+  // Without an observation time the rows could only be dated as new.
+  for (const fetchedAt of [undefined, null, '7'])
+    await assert.rejects(
+      createAdsbGnssSource({
+        fetchImpl: reply({ rows: [], fetchedAt }),
+      }).getSnapshot({ latitude: 1, longitude: 2 }),
+      /no observation time/,
+    );
 
   await assert.rejects(
     createAdsbGnssSource({ fetchImpl: reply({}) }).getSnapshot({}),