refactor(cleanup): Remove 4,500 lines of dead code (Phase 1 legacy cleanup)

- Delete engine/emitters.py (25 lines, unused Protocol definitions)
- Delete engine/beautiful_mermaid.py (4,107 lines, unused Mermaid ASCII renderer)
- Delete engine/pipeline_viz.py (364 lines, unused visualization module)
- Delete tests/test_emitters.py (orphaned test file)
- Remove introspect_pipeline_viz() method and references from engine/pipeline.py
- Add comprehensive legacy code analysis documentation in docs/

Phase 1 of legacy code cleanup: 0 risk, 100% safe to remove.
All tests pass (521 passing tests, 9 fewer due to test_emitters.py removal).
No regressions or breaking changes.

tests/test_emitters.py

@@ -1,69 +0,0 @@
-"""
-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_sensor_complies_with_protocol(self):
-        """MicSensor implements Startable and Stoppable protocols."""
-        from engine.sensors.mic import MicSensor
-
-        sensor = MicSensor()
-        assert hasattr(sensor, "start")
-        assert hasattr(sensor, "stop")
-        assert callable(sensor.start)
-        assert callable(sensor.stop)