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/fade.py

@@ -3,7 +3,7 @@ import random
 from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 
 
-class FadeEffect:
+class FadeEffect(EffectPlugin):
     name = "fade"
     config = EffectConfig(enabled=True, intensity=1.0)
 
@@ -54,5 +54,5 @@ class FadeEffect:
                 i += 1
         return "".join(result)
 
-    def configure(self, cfg: EffectConfig) -> None:
-        self.config = cfg
+    def configure(self, config: EffectConfig) -> None:
+        self.config = config