feat(app): add demo mode with HUD effect plugin

- Add --demo flag that runs effect showcase with pygame display
- Add HUD effect plugin (effects_plugins/hud.py) that displays:
  - FPS and frame time
  - Current effect name with intensity bar
  - Pipeline order
- Demo mode cycles through noise, fade, glitch, firehose effects
- Ramps intensity 0→1→0 over 5 seconds per effect

engine/config.py

@@ -241,6 +241,10 @@ DISPLAY = _arg_value("--display", sys.argv) or "terminal"
 WEBSOCKET = "--websocket" in sys.argv
 WEBSOCKET_PORT = _arg_int("--websocket-port", 8765)
 
+# ─── DEMO MODE ────────────────────────────────────────────
+DEMO = "--demo" in sys.argv
+DEMO_EFFECT_DURATION = 5.0  # seconds per effect
+
 
 def set_font_selection(font_path=None, font_index=None):
     """Set runtime primary font selection."""