Implement P05 checked services and safe selected delivery recovery

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a092fe-13b1-7f12-ac74-7d258af4d79c

src/user_engine/ports.py

@@ -437,6 +437,9 @@ class UserEngineStore(Protocol):
     def pending_outbox(self) -> tuple[OutboxEvent, ...]:
         """Return pending outbox events in write order."""
 
+    def outbox_delivery_guard(self, event_id: str) -> ContextManager[None]:
+        """Serialize delivery/replay for one event across processes."""
+
     def save_outbox(self, event: OutboxEvent) -> None:
         """Persist outbox delivery state."""