perf(kanban): stop board cards re-rendering on every agent event

Memoize the board so streaming events from an active run no longer force
all 38 collapsed cards to re-render — only the open CardModal/AgentRunBar
re-renders, since its props stay referentially equal.

- KanbanCard: wrap in React.memo; take isRunning/bevyRunning primitives
  instead of the whole orchestrator object; onOpen now takes the card id.
- useOrchestrator: expose a memoized activeByCard map, recomputed only
  when runs/bevyRunning change (not on every streamed text/tool event).
- KanbanBoardPage: pass a stable openById callback + primitive props so
  memo bails; memoize StatCard; extract the static category legend to a
  memoized CategoryLegend component.

Also drops the per-card .filter().find() status scans each render.

apps/docs/src/components/kanban/useOrchestrator.ts

@@ -53,6 +53,12 @@ export interface UseOrchestrator {
   bevyIsRunning: (runId: string) => boolean;
   /** Re-fetch a run's Bevy status from the server (truth after a reconnect). */
   refreshBevyStatus: (runId: string) => Promise<void>;
+  /**
+   * Active runs indexed by card id. Memoized and referentially stable unless
+   * the active set actually changes (not on every streamed event), so memoized
+   * card components can read their flags without re-rendering on noise.
+   */
+  activeByCard: Map<string, { running: boolean; bevy: boolean; runId: string }>;
 }
 
 export function useOrchestrator(): UseOrchestrator {
@@ -283,6 +289,21 @@ export function useOrchestrator(): UseOrchestrator {
 
   const bevyIsRunning = useCallback((runId: string) => bevyRunning.has(runId), [bevyRunning]);
 
+  /**
+   * Card-id index of active runs. Recomputed only when `runs` or `bevyRunning`
+   * changes — NOT on every streamed event — so memoized consumers stay stable.
+   * Replaces the per-card `.filter().find()` scans the board used to do.
+   */
+  const activeByCard = useMemo(() => {
+    const m = new Map<string, { running: boolean; bevy: boolean; runId: string }>();
+    for (const r of runs) {
+      if (r.status === 'running') {
+        m.set(r.cardId, { running: true, bevy: bevyRunning.has(r.id), runId: r.id });
+      }
+    }
+    return m;
+  }, [runs, bevyRunning]);
+
   const refreshBevyStatus = useCallback(async (runId: string) => {
     try {
       const { running } = await orchestratorApi.bevyStatus(runId);
@@ -317,5 +338,6 @@ export function useOrchestrator(): UseOrchestrator {
     stopBevy,
     bevyIsRunning,
     refreshBevyStatus,
+    activeByCard,
   };
 }