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/index.js

@@ -157,9 +157,11 @@ export function createGnssIntegrityLayer({
           return false;
         // Date the rows by when the proxy observed them: a cached or stale
         // replay keeps its original age instead of restarting the window.
-        accumulateGnssObservations(_observations, snapshot.rows, now(), {
+        // The proxy reports that age, so client and server clocks never mix.
+        const at = now();
+        accumulateGnssObservations(_observations, snapshot.rows, at, {
           windowMs,
-          observedAt: snapshot.fetchedAt,
+          observedAt: at - snapshot.ageMs,
         });
         _cells = binGnssCells(_observations);
         render();