perf(cluster): throttle the grid refreshCells to 200ms (coalesce RBN status bursts)

spotStatus updates ~20x/second under an RBN firehose; firing a full refreshCells
on each was pure churn on a slow PC. Coalesce into one refresh per 200ms. Keeps
the NEW/WORKED badge colours and the 'represents nothing' dimming — the dimming
itself is a trivial pure read now that the worked-index scan is cached, so it's
no longer a cost worth removing.

frontend/src/components/ClusterGrid.tsx

@@ -424,15 +424,22 @@ export function ClusterGrid({ rows, spotStatus, onSpotClick }: Props) {
   const context = useMemo(() => ({ spotStatus }), [spotStatus]);
 
   // Spot statuses arrive asynchronously (~after the rows render). The Call/Band/
-  // Mode cellStyles depend on them but their cell VALUE doesn't change, so ag-grid
-  // won't re-render those cells on its own — force a refresh so e.g. a worked call
-  // turns blue once its status loads.
+  // Mode cellStyles and the dimmed "represents nothing" class depend on them but
+  // the cell VALUE doesn't change, so ag-grid won't re-render on its own — force a
+  // refresh so e.g. a worked call turns blue once its status loads.
+  //
+  // THROTTLED: under an RBN firehose spotStatus updates ~20×/second, and firing a
+  // full refreshCells that often is pure churn on a slow PC. Coalesce the bursts
+  // into one refresh every 200 ms (still imperceptible) instead of one per update.
+  const refreshPending = useRef<number | undefined>(undefined);
   useEffect(() => {
-    // Light refresh so status-dependent styling — the Call/Band/Mode colours AND
-    // the dimmed "represents nothing" class (cellClassRules above) — re-applies
-    // once a status lands, WITHOUT redrawing whole rows (which pegged the CPU).
-    gridRef.current?.api?.refreshCells({ force: true });
+    if (refreshPending.current !== undefined) return; // a refresh is already queued
+    refreshPending.current = window.setTimeout(() => {
+      refreshPending.current = undefined;
+      gridRef.current?.api?.refreshCells({ force: true });
+    }, 200);
   }, [spotStatus]);
+  useEffect(() => () => { if (refreshPending.current !== undefined) window.clearTimeout(refreshPending.current); }, []);
 
   // Restore AFTER the profile scope is known — this grid has no key= remount to
   // save it from reading the wrong (unscoped) cache key at first paint.