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/engine/src/scheduler.ts

@@ -516,7 +516,7 @@ export class Scheduler {
       return;
     }
 
-    void this.store.listTasks({ slim: true, includeArchived: false })
+    void this.store.listTasks({ slim: true, includeArchived: false, startupMemo: true })
       .then((tasks) => {
         const repo = getCurrentRepo(this.store.getRootDir());
         if (!repo) return;