Add pubsub event system and integrate sync notifications

src/seedpass/core/pubsub.py

@@ -0,0 +1,27 @@
+from collections import defaultdict
+from typing import Callable, Dict, List, Any
+
+
+class PubSub:
+    """Simple in-process event bus using the observer pattern."""
+
+    def __init__(self) -> None:
+        self._subscribers: Dict[str, List[Callable[..., None]]] = defaultdict(list)
+
+    def subscribe(self, event: str, callback: Callable[..., None]) -> None:
+        """Register ``callback`` to be invoked when ``event`` is published."""
+        self._subscribers[event].append(callback)
+
+    def unsubscribe(self, event: str, callback: Callable[..., None]) -> None:
+        """Unregister ``callback`` from ``event`` notifications."""
+        if callback in self._subscribers.get(event, []):
+            self._subscribers[event].remove(callback)
+
+    def publish(self, event: str, *args: Any, **kwargs: Any) -> None:
+        """Notify all subscribers of ``event`` passing ``*args`` and ``**kwargs``."""
+        for callback in list(self._subscribers.get(event, [])):
+            callback(*args, **kwargs)
+
+
+# Global bus instance for convenience
+bus = PubSub()