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

tests/test_benchmark.py

@@ -37,8 +37,8 @@ class TestBenchmarkNullDisplay:
         """Effects should meet minimum processing throughput."""
         import time
 
-        from effects_plugins import discover_plugins
         from engine.effects import EffectContext, get_registry
+        from engine.effects.plugins import discover_plugins
 
         discover_plugins()
         registry = get_registry()