feat(tests): improve coverage to 56%, add benchmark regression tests

- Add EffectPlugin ABC with @abstractmethod decorators for interface enforcement
- Add runtime interface checking in discover_plugins() with issubclass()
- Add EffectContext factory with sensible defaults
- Standardize Display __init__ (remove redundant init in TerminalDisplay)
- Document effect behavior when ticker_height=0
- Evaluate legacy effects: document coexistence, no deprecation needed
- Research plugin patterns (VST, Python entry points)
- Fix pysixel dependency (removed broken dependency)

Test coverage improvements:
- Add DisplayRegistry tests
- Add MultiDisplay tests
- Add SixelDisplay tests
- Add controller._get_display tests
- Add effects controller command handling tests
- Add benchmark regression tests (@pytest.mark.benchmark)
- Add pytest marker for benchmark tests in pyproject.toml

Documentation updates:
- Update AGENTS.md with 56% coverage stats and effect plugin docs
- Update README.md with Sixel display mode and benchmark commands
- Add new modules to architecture section

tests/test_controller.py

@@ -5,7 +5,75 @@ Tests for engine.controller module.
 from unittest.mock import MagicMock, patch
 
 from engine import config
-from engine.controller import StreamController
+from engine.controller import StreamController, _get_display
+
+
+class TestGetDisplay:
+    """Tests for _get_display function."""
+
+    @patch("engine.controller.WebSocketDisplay")
+    @patch("engine.controller.TerminalDisplay")
+    def test_get_display_terminal(self, mock_terminal, mock_ws):
+        """returns TerminalDisplay for display=terminal."""
+        mock_terminal.return_value = MagicMock()
+        mock_ws.return_value = MagicMock()
+
+        cfg = config.Config(display="terminal")
+        display = _get_display(cfg)
+
+        mock_terminal.assert_called()
+        assert isinstance(display, MagicMock)
+
+    @patch("engine.controller.WebSocketDisplay")
+    @patch("engine.controller.TerminalDisplay")
+    def test_get_display_websocket(self, mock_terminal, mock_ws):
+        """returns WebSocketDisplay for display=websocket."""
+        mock_ws_instance = MagicMock()
+        mock_ws.return_value = mock_ws_instance
+        mock_terminal.return_value = MagicMock()
+
+        cfg = config.Config(display="websocket")
+        _get_display(cfg)
+
+        mock_ws.assert_called()
+        mock_ws_instance.start_server.assert_called()
+        mock_ws_instance.start_http_server.assert_called()
+
+    @patch("engine.controller.SixelDisplay")
+    def test_get_display_sixel(self, mock_sixel):
+        """returns SixelDisplay for display=sixel."""
+        mock_sixel.return_value = MagicMock()
+        cfg = config.Config(display="sixel")
+        _get_display(cfg)
+
+        mock_sixel.assert_called()
+
+    def test_get_display_unknown_returns_null(self):
+        """returns NullDisplay for unknown display mode."""
+        cfg = config.Config(display="unknown")
+        display = _get_display(cfg)
+
+        from engine.display import NullDisplay
+
+        assert isinstance(display, NullDisplay)
+
+    @patch("engine.controller.WebSocketDisplay")
+    @patch("engine.controller.TerminalDisplay")
+    @patch("engine.controller.MultiDisplay")
+    def test_get_display_both(self, mock_multi, mock_terminal, mock_ws):
+        """returns MultiDisplay for display=both."""
+        mock_terminal_instance = MagicMock()
+        mock_ws_instance = MagicMock()
+        mock_terminal.return_value = mock_terminal_instance
+        mock_ws.return_value = mock_ws_instance
+
+        cfg = config.Config(display="both")
+        _get_display(cfg)
+
+        mock_multi.assert_called()
+        call_args = mock_multi.call_args[0][0]
+        assert mock_terminal_instance in call_args
+        assert mock_ws_instance in call_args
 
 
 class TestStreamController:
@@ -68,6 +136,24 @@ class TestStreamController:
         assert mic_ok is False
         assert ntfy_ok is True
 
+    @patch("engine.controller.MicMonitor")
+    def test_initialize_sources_cc_subscribed(self, mock_mic):
+        """initialize_sources subscribes C&C handler."""
+        mock_mic_instance = MagicMock()
+        mock_mic_instance.available = False
+        mock_mic_instance.start.return_value = False
+        mock_mic.return_value = mock_mic_instance
+
+        with patch("engine.controller.NtfyPoller") as mock_ntfy:
+            mock_ntfy_instance = MagicMock()
+            mock_ntfy_instance.start.return_value = True
+            mock_ntfy.return_value = mock_ntfy_instance
+
+            controller = StreamController()
+            controller.initialize_sources()
+
+            mock_ntfy_instance.subscribe.assert_called()
+
 
 class TestStreamControllerCleanup:
     """Tests for StreamController cleanup."""