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]>

scripts/qa-gnss.mjs

@@ -250,7 +250,11 @@ async function stubRoute(page, { ageMs = 0 } = {}) {
         if (!url.includes('/api/gnss-integrity')) return realFetch(input, init);
         const ok = window.__gnssQaMode === 'fixture';
         const body = ok
-          ? { ...payload, fetchedAt: Date.now() - (window.__gnssQaAgeMs || 0) }
+          ? {
+              ...payload,
+              fetchedAt: Date.now() - (window.__gnssQaAgeMs || 0),
+              ageMs: window.__gnssQaAgeMs || 0,
+            }
           : { error: 'gnss_integrity_unavailable' };
         return Promise.resolve(
           new Response(JSON.stringify(body), {