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

.changeset/fn-4027-startup-listtasks-memo.md

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+---
+"@runfusion/fusion": patch
+---
+
+Memoize startup slim `listTasks` reads across dashboard/engine boot paths to reduce duplicate task-list SQL and JSON parsing work without introducing long-lived stale cache behavior.