refactor: phase 4 - event-driven architecture foundation

- Add EventBus class with pub/sub messaging (thread-safe)
- Add emitter Protocol classes (EventEmitter, Startable, Stoppable)
- Add event emission to NtfyPoller (NtfyMessageEvent)
- Add event emission to MicMonitor (MicLevelEvent)
- Update StreamController to publish stream start/end events
- Add comprehensive tests for eventbus and emitters modules

engine/eventbus.py

@@ -0,0 +1,72 @@
+"""
+Event bus - pub/sub messaging for decoupled component communication.
+"""
+
+import threading
+from collections import defaultdict
+from collections.abc import Callable
+from typing import Any
+
+from engine.events import EventType
+
+
+class EventBus:
+    """Thread-safe event bus for publish-subscribe messaging."""
+
+    def __init__(self):
+        self._subscribers: dict[EventType, list[Callable[[Any], None]]] = defaultdict(
+            list
+        )
+        self._lock = threading.Lock()
+
+    def subscribe(self, event_type: EventType, callback: Callable[[Any], None]) -> None:
+        """Register a callback for a specific event type."""
+        with self._lock:
+            self._subscribers[event_type].append(callback)
+
+    def unsubscribe(
+        self, event_type: EventType, callback: Callable[[Any], None]
+    ) -> None:
+        """Remove a callback for a specific event type."""
+        with self._lock:
+            if callback in self._subscribers[event_type]:
+                self._subscribers[event_type].remove(callback)
+
+    def publish(self, event_type: EventType, event: Any = None) -> None:
+        """Publish an event to all subscribers."""
+        with self._lock:
+            callbacks = list(self._subscribers.get(event_type, []))
+        for callback in callbacks:
+            try:
+                callback(event)
+            except Exception:
+                pass
+
+    def clear(self) -> None:
+        """Remove all subscribers."""
+        with self._lock:
+            self._subscribers.clear()
+
+    def subscriber_count(self, event_type: EventType | None = None) -> int:
+        """Get subscriber count for an event type, or total if None."""
+        with self._lock:
+            if event_type is None:
+                return sum(len(cb) for cb in self._subscribers.values())
+            return len(self._subscribers.get(event_type, []))
+
+
+_event_bus: EventBus | None = None
+
+
+def get_event_bus() -> EventBus:
+    """Get the global event bus instance."""
+    global _event_bus
+    if _event_bus is None:
+        _event_bus = EventBus()
+    return _event_bus
+
+
+def set_event_bus(bus: EventBus) -> None:
+    """Set the global event bus instance (for testing)."""
+    global _event_bus
+    _event_bus = bus