Outages review fixes: reset on new target, memoize marker icons, coverage note in popup (#184)

- useOutagesIn kept the previous incident's answer while fetching the next
  one (IncidentPanel isn't keyed), so a newly selected incident briefly read
  'none reported here'. Drop the old answer when the target changes.
- OutageLayer built a fresh divIcon per marker every render; MapView
  re-renders on live updates, so Leaflet rebuilt every marker's DOM. Memoize.
- Popup now carries the provider's coverage note (ODIN: non-reporting
  utilities, e.g. Con Ed/O&R, show nothing even during an outage).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/lib/useOutages.ts

@@ -34,7 +34,10 @@ function useOutageFetch(
   const seq = useRef(0);
 
   useEffect(() => {
-    if (!getBox) { setData(null); setError(null); setLoaded(false); return; }
+    // A new target (another incident, overlay re-enabled) must not be judged
+    // against the previous target's answer: drop it and show "checking…".
+    setData(null); setError(null); setLoaded(false);
+    if (!getBox) return;
     const boxFn = getBox;
     let last = 0;
     const load = async () => {