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_hud.py

@@ -1,11 +1,10 @@
-
 from engine.effects.performance import PerformanceMonitor, set_monitor
 from engine.effects.types import EffectContext
 
 
 def test_hud_effect_adds_hud_lines():
     """Test that HUD effect adds HUD lines to the buffer."""
-    from effects_plugins.hud import HudEffect
+    from engine.effects.plugins.hud import HudEffect
 
     set_monitor(PerformanceMonitor())
 
@@ -51,7 +50,7 @@ def test_hud_effect_adds_hud_lines():
 
 def test_hud_effect_shows_current_effect():
     """Test that HUD displays the correct effect name."""
-    from effects_plugins.hud import HudEffect
+    from engine.effects.plugins.hud import HudEffect
 
     set_monitor(PerformanceMonitor())
 
@@ -80,7 +79,7 @@ def test_hud_effect_shows_current_effect():
 
 def test_hud_effect_shows_intensity():
     """Test that HUD displays intensity percentage."""
-    from effects_plugins.hud import HudEffect
+    from engine.effects.plugins.hud import HudEffect
 
     set_monitor(PerformanceMonitor())