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_sixel.py

@@ -0,0 +1,123 @@
+"""
+Tests for engine.display.backends.sixel module.
+"""
+
+from unittest.mock import MagicMock, patch
+
+
+class TestSixelDisplay:
+    """Tests for SixelDisplay class."""
+
+    def test_init_stores_dimensions(self):
+        """init stores dimensions."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+        display.init(80, 24)
+        assert display.width == 80
+        assert display.height == 24
+
+    def test_init_custom_cell_size(self):
+        """init accepts custom cell size."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay(cell_width=12, cell_height=18)
+        assert display.cell_width == 12
+        assert display.cell_height == 18
+
+    def test_show_handles_empty_buffer(self):
+        """show handles empty buffer gracefully."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+        display.init(80, 24)
+
+        with patch("engine.display.backends.sixel._encode_sixel") as mock_encode:
+            mock_encode.return_value = ""
+            display.show([])
+
+    def test_show_handles_pil_import_error(self):
+        """show gracefully handles missing PIL."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+        display.init(80, 24)
+
+        with patch.dict("sys.modules", {"PIL": None}):
+            display.show(["test line"])
+
+    def test_clear_sends_escape_sequence(self):
+        """clear sends clear screen escape sequence."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+
+        with patch("sys.stdout") as mock_stdout:
+            display.clear()
+            mock_stdout.buffer.write.assert_called()
+
+    def test_cleanup_does_nothing(self):
+        """cleanup does nothing."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+        display.cleanup()
+
+
+class TestSixelAnsiParsing:
+    """Tests for ANSI parsing in SixelDisplay."""
+
+    def test_parse_empty_string(self):
+        """handles empty string."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("")
+        assert len(result) > 0
+
+    def test_parse_plain_text(self):
+        """parses plain text without ANSI codes."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("hello world")
+        assert len(result) == 1
+        text, fg, bg, bold = result[0]
+        assert text == "hello world"
+
+    def test_parse_with_color_codes(self):
+        """parses ANSI color codes."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("\033[31mred\033[0m")
+        assert len(result) == 2
+
+    def test_parse_with_bold(self):
+        """parses bold codes."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("\033[1mbold\033[0m")
+        assert len(result) == 2
+
+    def test_parse_256_color(self):
+        """parses 256 color codes."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("\033[38;5;196mred\033[0m")
+        assert len(result) == 2
+
+
+class TestSixelEncoding:
+    """Tests for Sixel encoding."""
+
+    def test_encode_empty_image(self):
+        """handles empty image."""
+        from engine.display.backends.sixel import _encode_sixel
+
+        with patch("PIL.Image.Image") as mock_image:
+            mock_img_instance = MagicMock()
+            mock_img_instance.convert.return_value = mock_img_instance
+            mock_img_instance.size = (0, 0)
+            mock_img_instance.load.return_value = {}
+            mock_image.return_value = mock_img_instance
+
+            result = _encode_sixel(mock_img_instance)
+            assert result == ""