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_effects_controller.py

@@ -5,8 +5,10 @@ Tests for engine.effects.controller module.
 from unittest.mock import MagicMock, patch
 
 from engine.effects.controller import (
+    _format_stats,
     handle_effects_command,
     set_effect_chain_ref,
+    show_effects_menu,
 )
 
 
@@ -92,6 +94,29 @@ class TestHandleEffectsCommand:
                 assert "Reordered pipeline" in result
                 mock_chain_instance.reorder.assert_called_once_with(["noise", "fade"])
 
+    def test_reorder_failure(self):
+        """reorder returns error on failure."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_registry.return_value.list_all.return_value = {}
+
+            with patch("engine.effects.controller._get_effect_chain") as mock_chain:
+                mock_chain_instance = MagicMock()
+                mock_chain_instance.reorder.return_value = False
+                mock_chain.return_value = mock_chain_instance
+
+                result = handle_effects_command("/effects reorder bad")
+
+                assert "Failed to reorder" in result
+
+    def test_unknown_effect(self):
+        """unknown effect returns error."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_registry.return_value.list_all.return_value = {}
+
+            result = handle_effects_command("/effects unknown on")
+
+            assert "Unknown effect" in result
+
     def test_unknown_command(self):
         """unknown command returns error."""
         result = handle_effects_command("/unknown")
@@ -102,6 +127,105 @@ class TestHandleEffectsCommand:
         result = handle_effects_command("not a command")
         assert "Unknown command" in result
 
+    def test_invalid_intensity_value(self):
+        """invalid intensity value returns error."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_plugin = MagicMock()
+            mock_registry.return_value.get.return_value = mock_plugin
+            mock_registry.return_value.list_all.return_value = {"noise": mock_plugin}
+
+            result = handle_effects_command("/effects noise intensity bad")
+
+            assert "Invalid intensity" in result
+
+    def test_missing_action(self):
+        """missing action returns usage."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_plugin = MagicMock()
+            mock_registry.return_value.get.return_value = mock_plugin
+            mock_registry.return_value.list_all.return_value = {"noise": mock_plugin}
+
+            result = handle_effects_command("/effects noise")
+
+            assert "Usage" in result
+
+    def test_stats_command(self):
+        """stats command returns formatted stats."""
+        with patch("engine.effects.controller.get_monitor") as mock_monitor:
+            mock_monitor.return_value.get_stats.return_value = {
+                "frame_count": 100,
+                "pipeline": {"avg_ms": 1.5, "min_ms": 1.0, "max_ms": 2.0},
+                "effects": {},
+            }
+
+            result = handle_effects_command("/effects stats")
+
+            assert "Performance Stats" in result
+
+    def test_list_only_effects(self):
+        """list command works with just /effects."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_plugin = MagicMock()
+            mock_plugin.config.enabled = False
+            mock_plugin.config.intensity = 0.5
+            mock_registry.return_value.list_all.return_value = {"noise": mock_plugin}
+
+            with patch("engine.effects.controller._get_effect_chain") as mock_chain:
+                mock_chain.return_value = None
+
+                result = handle_effects_command("/effects")
+
+                assert "noise: OFF" in result
+
+
+class TestShowEffectsMenu:
+    """Tests for show_effects_menu function."""
+
+    def test_returns_formatted_menu(self):
+        """returns formatted effects menu."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_plugin = MagicMock()
+            mock_plugin.config.enabled = True
+            mock_plugin.config.intensity = 0.75
+            mock_registry.return_value.list_all.return_value = {"noise": mock_plugin}
+
+            with patch("engine.effects.controller._get_effect_chain") as mock_chain:
+                mock_chain_instance = MagicMock()
+                mock_chain_instance.get_order.return_value = ["noise"]
+                mock_chain.return_value = mock_chain_instance
+
+                result = show_effects_menu()
+
+                assert "EFFECTS MENU" in result
+                assert "noise" in result
+
+
+class TestFormatStats:
+    """Tests for _format_stats function."""
+
+    def test_returns_error_when_no_monitor(self):
+        """returns error when monitor unavailable."""
+        with patch("engine.effects.controller.get_monitor") as mock_monitor:
+            mock_monitor.return_value.get_stats.return_value = {"error": "No data"}
+
+            result = _format_stats()
+
+            assert "No data" in result
+
+    def test_formats_pipeline_stats(self):
+        """formats pipeline stats correctly."""
+        with patch("engine.effects.controller.get_monitor") as mock_monitor:
+            mock_monitor.return_value.get_stats.return_value = {
+                "frame_count": 50,
+                "pipeline": {"avg_ms": 2.5, "min_ms": 2.0, "max_ms": 3.0},
+                "effects": {"noise": {"avg_ms": 0.5, "min_ms": 0.4, "max_ms": 0.6}},
+            }
+
+            result = _format_stats()
+
+            assert "Pipeline" in result
+            assert "noise" in result
+
 
 class TestSetEffectChainRef:
     """Tests for set_effect_chain_ref function."""