feat(FN-5241): add atomic in-review handoff seam with executor/self-healing

The merge introduces an atomic review handoff seam in the core store (`packages/core/src/store.ts`) and migrates executor and self-healing transitions to use it, replacing the previous multi-step mutable-state handoff with a single transactional operation. Extensive reliability backstops and regress

Fusion-Task-Id: FN-5241

packages/engine/src/__tests__/executor-test-helpers.ts

@@ -321,6 +321,7 @@ export function createMockStore() {
     }),
     updateTask: vi.fn().mockResolvedValue({}),
     moveTask: vi.fn().mockResolvedValue({}),
+    handoffToReview: vi.fn().mockImplementation(async (id: string) => store.moveTask(id, "in-review")),
     mergeTask: vi.fn().mockResolvedValue({}),
     createTask: vi.fn().mockImplementation(async (input: Record<string, unknown>) => ({
       id: "FN-002",