Add public registration cancellation

src/user_engine/ports.py

@@ -181,6 +181,9 @@ class RegistrationVerificationPort(Protocol):
     def consume(self, opaque_handle: str) -> VerifiedRegistrationApplicant:
         """Atomically consume verified, unexpired applicant evidence."""
 
+    def cancel(self, opaque_handle: str) -> VerifiedRegistrationApplicant:
+        """Atomically cancel an unexpired applicant intent using mailbox evidence."""
+
 
 class UserEngineStore(Protocol):
     """Durable persistence boundary for user-engine service behavior.