feat(FN-4027): memoize startup slim list reads to avoid redundant fresh fet

Added memoization for startup slim task list reads (`store.ts`), with dashboard server routing startup reads through the memo window and tests covering the store watcher consumers in scheduler and worktree-pool, plus performance documentation.

Fusion-Task-Id: FN-4027

packages/dashboard/src/server.ts

@@ -1581,7 +1581,7 @@ export function setupBadgeWebSocket(
   };
 
   // Prime cache with existing tasks from default store
-  void store.listTasks({ slim: true, includeArchived: false }).then((tasks) => {
+  void store.listTasks({ slim: true, includeArchived: false, startupMemo: true }).then((tasks) => {
     for (const task of tasks) {
       badgeSnapshots.set(`default:${task.id}`, {
         prInfo: task.prInfo ?? null,