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_benchmark.py

@@ -0,0 +1,100 @@
+"""
+Tests for engine.benchmark module - performance regression tests.
+"""
+
+from unittest.mock import patch
+
+import pytest
+
+from engine.display import NullDisplay
+
+
+class TestBenchmarkNullDisplay:
+    """Performance tests for NullDisplay - regression tests."""
+
+    @pytest.mark.benchmark
+    def test_null_display_minimum_fps(self):
+        """NullDisplay should meet minimum performance threshold."""
+        import time
+
+        display = NullDisplay()
+        display.init(80, 24)
+        buffer = ["x" * 80 for _ in range(24)]
+
+        iterations = 1000
+        start = time.perf_counter()
+        for _ in range(iterations):
+            display.show(buffer)
+        elapsed = time.perf_counter() - start
+
+        fps = iterations / elapsed
+        min_fps = 20000
+
+        assert fps >= min_fps, f"NullDisplay FPS {fps:.0f} below minimum {min_fps}"
+
+    @pytest.mark.benchmark
+    def test_effects_minimum_throughput(self):
+        """Effects should meet minimum processing throughput."""
+        import time
+
+        from effects_plugins import discover_plugins
+        from engine.effects import EffectContext, get_registry
+
+        discover_plugins()
+        registry = get_registry()
+        effect = registry.get("noise")
+        assert effect is not None, "Noise effect should be registered"
+
+        buffer = ["x" * 80 for _ in range(24)]
+        ctx = EffectContext(
+            terminal_width=80,
+            terminal_height=24,
+            scroll_cam=0,
+            ticker_height=20,
+            mic_excess=0.0,
+            grad_offset=0.0,
+            frame_number=0,
+            has_message=False,
+        )
+
+        iterations = 500
+        start = time.perf_counter()
+        for _ in range(iterations):
+            effect.process(buffer, ctx)
+        elapsed = time.perf_counter() - start
+
+        fps = iterations / elapsed
+        min_fps = 10000
+
+        assert fps >= min_fps, (
+            f"Effect processing FPS {fps:.0f} below minimum {min_fps}"
+        )
+
+
+class TestBenchmarkWebSocketDisplay:
+    """Performance tests for WebSocketDisplay."""
+
+    @pytest.mark.benchmark
+    def test_websocket_display_minimum_fps(self):
+        """WebSocketDisplay should meet minimum performance threshold."""
+        import time
+
+        with patch("engine.display.backends.websocket.websockets", None):
+            from engine.display import WebSocketDisplay
+
+            display = WebSocketDisplay()
+            display.init(80, 24)
+            buffer = ["x" * 80 for _ in range(24)]
+
+            iterations = 500
+            start = time.perf_counter()
+            for _ in range(iterations):
+                display.show(buffer)
+            elapsed = time.perf_counter() - start
+
+            fps = iterations / elapsed
+            min_fps = 10000
+
+            assert fps >= min_fps, (
+                f"WebSocketDisplay FPS {fps:.0f} below minimum {min_fps}"
+            )