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_display.py

@@ -2,7 +2,10 @@
 Tests for engine.display module.
 """
 
-from engine.display import NullDisplay, TerminalDisplay
+from unittest.mock import MagicMock
+
+from engine.display import DisplayRegistry, NullDisplay, TerminalDisplay
+from engine.display.backends.multi import MultiDisplay
 
 
 class TestDisplayProtocol:
@@ -25,6 +28,66 @@ class TestDisplayProtocol:
         assert hasattr(display, "cleanup")
 
 
+class TestDisplayRegistry:
+    """Tests for DisplayRegistry class."""
+
+    def setup_method(self):
+        """Reset registry before each test."""
+        DisplayRegistry._backends = {}
+        DisplayRegistry._initialized = False
+
+    def test_register_adds_backend(self):
+        """register adds a backend to the registry."""
+        DisplayRegistry.register("test", TerminalDisplay)
+        assert DisplayRegistry.get("test") == TerminalDisplay
+
+    def test_register_case_insensitive(self):
+        """register is case insensitive."""
+        DisplayRegistry.register("TEST", TerminalDisplay)
+        assert DisplayRegistry.get("test") == TerminalDisplay
+
+    def test_get_returns_none_for_unknown(self):
+        """get returns None for unknown backend."""
+        assert DisplayRegistry.get("unknown") is None
+
+    def test_list_backends_returns_all(self):
+        """list_backends returns all registered backends."""
+        DisplayRegistry.register("a", TerminalDisplay)
+        DisplayRegistry.register("b", NullDisplay)
+        backends = DisplayRegistry.list_backends()
+        assert "a" in backends
+        assert "b" in backends
+
+    def test_create_returns_instance(self):
+        """create returns a display instance."""
+        DisplayRegistry.register("test", NullDisplay)
+        display = DisplayRegistry.create("test")
+        assert isinstance(display, NullDisplay)
+
+    def test_create_returns_none_for_unknown(self):
+        """create returns None for unknown backend."""
+        display = DisplayRegistry.create("unknown")
+        assert display is None
+
+    def test_initialize_registers_defaults(self):
+        """initialize registers default backends."""
+        DisplayRegistry.initialize()
+        assert DisplayRegistry.get("terminal") == TerminalDisplay
+        assert DisplayRegistry.get("null") == NullDisplay
+        from engine.display.backends.sixel import SixelDisplay
+        from engine.display.backends.websocket import WebSocketDisplay
+
+        assert DisplayRegistry.get("websocket") == WebSocketDisplay
+        assert DisplayRegistry.get("sixel") == SixelDisplay
+
+    def test_initialize_idempotent(self):
+        """initialize can be called multiple times safely."""
+        DisplayRegistry.initialize()
+        DisplayRegistry._backends["custom"] = TerminalDisplay
+        DisplayRegistry.initialize()
+        assert "custom" in DisplayRegistry.list_backends()
+
+
 class TestTerminalDisplay:
     """Tests for TerminalDisplay class."""
 
@@ -77,3 +140,62 @@ class TestNullDisplay:
         """cleanup does nothing."""
         display = NullDisplay()
         display.cleanup()
+
+
+class TestMultiDisplay:
+    """Tests for MultiDisplay class."""
+
+    def test_init_stores_dimensions(self):
+        """init stores dimensions and forwards to displays."""
+        mock_display1 = MagicMock()
+        mock_display2 = MagicMock()
+        multi = MultiDisplay([mock_display1, mock_display2])
+
+        multi.init(120, 40)
+
+        assert multi.width == 120
+        assert multi.height == 40
+        mock_display1.init.assert_called_once_with(120, 40)
+        mock_display2.init.assert_called_once_with(120, 40)
+
+    def test_show_forwards_to_all_displays(self):
+        """show forwards buffer to all displays."""
+        mock_display1 = MagicMock()
+        mock_display2 = MagicMock()
+        multi = MultiDisplay([mock_display1, mock_display2])
+
+        buffer = ["line1", "line2"]
+        multi.show(buffer)
+
+        mock_display1.show.assert_called_once_with(buffer)
+        mock_display2.show.assert_called_once_with(buffer)
+
+    def test_clear_forwards_to_all_displays(self):
+        """clear forwards to all displays."""
+        mock_display1 = MagicMock()
+        mock_display2 = MagicMock()
+        multi = MultiDisplay([mock_display1, mock_display2])
+
+        multi.clear()
+
+        mock_display1.clear.assert_called_once()
+        mock_display2.clear.assert_called_once()
+
+    def test_cleanup_forwards_to_all_displays(self):
+        """cleanup forwards to all displays."""
+        mock_display1 = MagicMock()
+        mock_display2 = MagicMock()
+        multi = MultiDisplay([mock_display1, mock_display2])
+
+        multi.cleanup()
+
+        mock_display1.cleanup.assert_called_once()
+        mock_display2.cleanup.assert_called_once()
+
+    def test_empty_displays_list(self):
+        """handles empty displays list gracefully."""
+        multi = MultiDisplay([])
+        multi.init(80, 24)
+        multi.show(["test"])
+        multi.clear()
+        multi.cleanup()