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/emitters.py

@@ -0,0 +1,25 @@
+"""
+Event emitter protocols - abstract interfaces for event-producing components.
+"""
+
+from collections.abc import Callable
+from typing import Any, Protocol
+
+
+class EventEmitter(Protocol):
+    """Protocol for components that emit events."""
+
+    def subscribe(self, callback: Callable[[Any], None]) -> None: ...
+    def unsubscribe(self, callback: Callable[[Any], None]) -> None: ...
+
+
+class Startable(Protocol):
+    """Protocol for components that can be started."""
+
+    def start(self) -> Any: ...
+
+
+class Stoppable(Protocol):
+    """Protocol for components that can be stopped."""
+
+    def stop(self) -> None: ...