refactor: move effects_plugins to engine/effects/plugins

- Move effects_plugins/ to engine/effects/plugins/
- Update imports in engine/app.py
- Update imports in all test files
- Follows capability-based deps architecture

Closes #27

engine/app.py

@@ -5,7 +5,7 @@ Application orchestrator — pipeline mode entry point.
 import sys
 import time
 
-import effects_plugins
+import engine.effects.plugins as effects_plugins
 from engine import config
 from engine.display import DisplayRegistry
 from engine.effects import PerformanceMonitor, get_registry, set_monitor