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.

docs/CURRENT-STATE.md

@@ -3072,7 +3072,7 @@ its criteria cannot be silently ignored.
 | GNSS Integrity 📡 (`gnss-interference`) | adsb.lol point snapshot, low navigation accuracy binned per 0.5° cell (GEV NIC/NACp threshold + gpsjam.org formula and bands, keyless, ODbL) | `src/layers/gnss/` via `src/app/layers/gnss.js` | `/api/gnss-integrity` | 60s (server cache 60 s; 30 min rolling window) |
 | Fire Perimeters 🔥 | NIFC WFIGS current interagency perimeters (keyless, paged past the 2000-record cap); InciWeb catalog + incident page origin and update time checks for verified incident-page links | `src/layers/perimeters/` via `src/app/layers/perimeters.js` | `/api/fire-perimeters` + `/api/fire-perimeters/inciweb/*` | 5 min (server caches: catalog 1 h; publication 30 min) |
 
-GNSS Integrity fetches a capped adsb.lol snapshot around the view anchor, keeps each aircraft once per cell over 30 minutes as the camera pans, and draws low/medium/high ground rectangles only for cells with at least 3 aircraft. The row legend counts cells per band. The method is a GEV threshold plus gpsjam.org aggregation, not the gpsjam method: the per-aircraft rule (NIC < 7 or NACp < 8, borrowed from the US 14 CFR 91.227 ADS-B Out minima, or readsb's GPS-loss flag) and the 30-minute visited-view window are GEV-defined and unvalidated; only the cell formula and 2% / 10% bands come from gpsjam.org. `layer.provenance` (also on `getStats()`) records each part, and the proxy tags its rows with `classifier: gev-nic-nacp-v1`. Cells carry `state: navigation-integrity-anomaly` (medium/high) or `nominal`; `interpretation: suspected-gnss-interference` is a secondary reading, not a detection or a located emitter.
+GNSS Integrity fetches a capped adsb.lol snapshot around the view anchor, keeps each aircraft once per cell over 30 minutes as the camera pans, and draws low/medium/high ground rectangles only for cells with at least 3 aircraft. The row legend counts cells per band. The method is a GEV threshold plus gpsjam.org aggregation, not the gpsjam method: the per-aircraft rule (NIC < 7 or NACp < 8, borrowed from the US 14 CFR 91.227 ADS-B Out minima, or readsb's GPS-loss flag) and the 30-minute visited-view window are GEV-defined and unvalidated; only the cell formula and 2% / 10% bands come from gpsjam.org. `layer.provenance` (also on `getStats()`) records each part, and the proxy tags its rows with `classifier: gev-nic-nacp-v1`. Cells carry `state: navigation-integrity-anomaly` (medium/high) or `nominal`; `interpretation: suspected-gnss-interference` is a secondary reading, not a detection or a located emitter. Only DO-260A/B (ADS-B versions 1 and 2) direct 1090ES positions with a known `seen_pos` of 0–60 s count. Presence and degraded evidence age separately: a degraded report keeps the aircraft degraded in its cell for 30 minutes after that report, however many healthy reports follow, and an aircraft unheard for 30 minutes starts fresh. Rows are dated by the snapshot's `fetchedAt`, which the proxy keeps on cached and stale answers, so a replayed snapshot never restarts the window; the proxy stops serving a stale snapshot once it is older than 30 minutes, the source refuses an undated one, and cells age out on schedule during fetch failures or while there is no view anchor.
 
 Fire Perimeters uses capped, timed server reads with stale-on-error caching and a per-client limit. Unchanged snapshots retain geometry; link checks cancel on disable or selection change, and the row legend shows reported containment.