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

engine/effects/__init__.py

@@ -10,7 +10,12 @@ from engine.effects.legacy import (
 )
 from engine.effects.performance import PerformanceMonitor, get_monitor, set_monitor
 from engine.effects.registry import EffectRegistry, get_registry, set_registry
-from engine.effects.types import EffectConfig, EffectContext, PipelineConfig
+from engine.effects.types import (
+    EffectConfig,
+    EffectContext,
+    PipelineConfig,
+    create_effect_context,
+)
 
 
 def get_effect_chain():
@@ -25,6 +30,7 @@ __all__ = [
     "EffectConfig",
     "EffectContext",
     "PipelineConfig",
+    "create_effect_context",
     "get_registry",
     "set_registry",
     "get_effect_chain",