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

effects_plugins/__init__.py

@@ -5,6 +5,7 @@ PLUGIN_DIR = Path(__file__).parent
 
 def discover_plugins():
     from engine.effects.registry import get_registry
+    from engine.effects.types import EffectPlugin
 
     registry = get_registry()
     imported = {}
@@ -22,11 +23,13 @@ def discover_plugins():
                 attr = getattr(module, attr_name)
                 if (
                     isinstance(attr, type)
-                    and hasattr(attr, "name")
-                    and hasattr(attr, "process")
+                    and issubclass(attr, EffectPlugin)
+                    and attr is not EffectPlugin
                     and attr_name.endswith("Effect")
                 ):
                     plugin = attr()
+                    if not isinstance(plugin, EffectPlugin):
+                        continue
                     registry.register(plugin)
                     imported[plugin.name] = plugin
         except Exception: