fix(arch-014,arch-020): no contract lazy-resurrect after unregister; event bus dedupes handlers

Also fixes ARCH-029/041: none-check before attribute access in trigger dispatcher.

app/core/event_bus.py

@@ -36,8 +36,13 @@ class EventBus:
         self._handlers: dict[str, list[EventHandler]] = defaultdict(list)
 
     def subscribe(self, event_name: str, handler: EventHandler) -> None:
-        """Subscribe a handler to an event."""
-        self._handlers[event_name].append(handler)
+        """Subscribe a handler to an event.
+
+        Idempotent: subscribing the same handler twice is a no-op
+        (ARCH-020) so double activation cannot fire handlers twice.
+        """
+        if handler not in self._handlers[event_name]:
+            self._handlers[event_name].append(handler)
 
     def unsubscribe(self, event_name: str, handler: EventHandler) -> None:
         """Unsubscribe a handler from an event."""