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

tests/test_emitters.py

@@ -0,0 +1,69 @@
+"""
+Tests for engine.emitters module.
+"""
+
+from engine.emitters import EventEmitter, Startable, Stoppable
+
+
+class TestEventEmitterProtocol:
+    """Tests for EventEmitter protocol."""
+
+    def test_protocol_exists(self):
+        """EventEmitter protocol is defined."""
+        assert EventEmitter is not None
+
+    def test_protocol_has_subscribe_method(self):
+        """EventEmitter has subscribe method in protocol."""
+        assert hasattr(EventEmitter, "subscribe")
+
+    def test_protocol_has_unsubscribe_method(self):
+        """EventEmitter has unsubscribe method in protocol."""
+        assert hasattr(EventEmitter, "unsubscribe")
+
+
+class TestStartableProtocol:
+    """Tests for Startable protocol."""
+
+    def test_protocol_exists(self):
+        """Startable protocol is defined."""
+        assert Startable is not None
+
+    def test_protocol_has_start_method(self):
+        """Startable has start method in protocol."""
+        assert hasattr(Startable, "start")
+
+
+class TestStoppableProtocol:
+    """Tests for Stoppable protocol."""
+
+    def test_protocol_exists(self):
+        """Stoppable protocol is defined."""
+        assert Stoppable is not None
+
+    def test_protocol_has_stop_method(self):
+        """Stoppable has stop method in protocol."""
+        assert hasattr(Stoppable, "stop")
+
+
+class TestProtocolCompliance:
+    """Tests that existing classes comply with protocols."""
+
+    def test_ntfy_poller_complies_with_protocol(self):
+        """NtfyPoller implements EventEmitter protocol."""
+        from engine.ntfy import NtfyPoller
+
+        poller = NtfyPoller("http://example.com/topic")
+        assert hasattr(poller, "subscribe")
+        assert hasattr(poller, "unsubscribe")
+        assert callable(poller.subscribe)
+        assert callable(poller.unsubscribe)
+
+    def test_mic_monitor_complies_with_protocol(self):
+        """MicMonitor implements EventEmitter and Startable protocols."""
+        from engine.mic import MicMonitor
+
+        monitor = MicMonitor()
+        assert hasattr(monitor, "subscribe")
+        assert hasattr(monitor, "unsubscribe")
+        assert hasattr(monitor, "start")
+        assert hasattr(monitor, "stop")