fix(traffic): the traffic layer reads STALE, not LIVE, on cached flow tiles

Once the TomTom tile budget is spent, or TomTom fails, the proxy serves
cached tiles marked STALE-BUDGET / STALE-ERROR. flowSource works out
`stale`, but the layer fetched flow through fetchFlowForBounds, which
returns only the segments, so getStats() and the sync chip still read
"LIVE · Roads: …". #1239 fixed the get_traffic_flow tool the same way; the
layer now uses fetchFlowDetail too, reports stale and oldestTileAt in its
stats, and the chip reads "STALE · Roads: <source> · Flow: cached TomTom
tiles". A source without fetchFlowDetail keeps the previous behaviour.

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

src/layers/traffic/flow.js

@@ -186,10 +186,19 @@ export function createFlow({ state: layerState, services, parts, source }) {
       return layerState._warmFlow;
     layerState._warmFlowGeneration = generation;
     const signal = layerState._activeFetchAbort?.signal;
-    layerState._warmFlow = ensureFlowStatus(signal).then(() => {
+    layerState._warmFlow = ensureFlowStatus(signal).then(async () => {
       if (!layerState._liveMode || generation !== layerState._loadGeneration)
         return [];
-      return fetchFlowForBounds(clamped, { signal });
+      if (typeof source.fetchFlowDetail !== 'function')
+        return fetchFlowForBounds(clamped, { signal });
+      // The proxy serves cached tiles once the TomTom budget is spent or the
+      // upstream fails; carry that mark so getStats() reads STALE, not LIVE.
+      const detail = await source.fetchFlowDetail(clamped, { signal });
+      if (generation === layerState._loadGeneration) {
+        layerState._flowStale = detail.stale === true;
+        layerState._flowOldestTileAt = detail.oldestTileAt ?? null;
+      }
+      return detail.segments;
     });
     layerState._warmFlow.catch(() => {});
     return layerState._warmFlow;