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/app.py

@@ -351,7 +351,128 @@ def pick_effects_config():
         termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)
 
 
+def run_demo_mode():
+    """Run demo mode - showcases effects with pygame display."""
+    import random
+
+    from engine import config
+    from engine.display import DisplayRegistry
+    from engine.effects import (
+        EffectContext,
+        PerformanceMonitor,
+        get_effect_chain,
+        get_registry,
+        set_monitor,
+    )
+
+    print("  \033[1;38;5;46mMAINLINE DEMO MODE\033[0m")
+    print("  \033[38;5;245mInitializing pygame display...\033[0m")
+
+    import effects_plugins
+
+    effects_plugins.discover_plugins()
+
+    registry = get_registry()
+    chain = get_effect_chain()
+    chain.set_order(["hud"])
+
+    monitor = PerformanceMonitor()
+    set_monitor(monitor)
+    chain._monitor = monitor
+
+    display = DisplayRegistry.create("pygame")
+    if not display:
+        print("  \033[38;5;196mFailed to create pygame display\033[0m")
+        sys.exit(1)
+
+    display.init(80, 24)
+    display.clear()
+
+    effects_to_demo = ["noise", "fade", "glitch", "firehose"]
+    w, h = 80, 24
+
+    base_buffer = []
+    for row in range(h):
+        line = ""
+        for col in range(w):
+            char = random.choice("ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 ")
+            line += char
+        base_buffer.append(line)
+
+    print("  \033[38;5;82mStarting effect demo...\033[0m")
+    print("  \033[38;5;245mPress Ctrl+C to exit\033[0m\n")
+
+    effect_idx = 0
+    effect_name = effects_to_demo[effect_idx]
+    effect_start_time = time.time()
+    current_intensity = 0.0
+    ramping_up = True
+
+    frame_count = 0
+
+    try:
+        while True:
+            elapsed = time.time() - effect_start_time
+            duration = config.DEMO_EFFECT_DURATION
+
+            if elapsed >= duration:
+                effect_idx = (effect_idx + 1) % len(effects_to_demo)
+                effect_name = effects_to_demo[effect_idx]
+                effect_start_time = time.time()
+                elapsed = 0
+                current_intensity = 0.0
+                ramping_up = True
+
+            progress = elapsed / duration
+            if ramping_up:
+                current_intensity = progress
+                if progress >= 1.0:
+                    ramping_up = False
+            else:
+                current_intensity = 1.0 - progress
+
+            for effect in registry.list_all().values():
+                if effect.name == effect_name:
+                    effect.config.enabled = True
+                    effect.config.intensity = current_intensity
+                elif effect.name not in ("hud", effect_name):
+                    effect.config.enabled = False
+
+            hud_effect = registry.get("hud")
+            if hud_effect:
+                hud_effect.config.params["display_effect"] = effect_name
+                hud_effect.config.params["display_intensity"] = current_intensity
+
+            ctx = EffectContext(
+                terminal_width=w,
+                terminal_height=h,
+                scroll_cam=0,
+                ticker_height=h,
+                mic_excess=0.0,
+                grad_offset=time.time() % 1.0,
+                frame_number=frame_count,
+                has_message=False,
+                items=[],
+            )
+
+            result = chain.process(base_buffer, ctx)
+            display.show(result)
+
+            frame_count += 1
+            time.sleep(1 / 60)
+
+    except KeyboardInterrupt:
+        pass
+    finally:
+        display.cleanup()
+        print("\n  \033[38;5;245mDemo ended\033[0m")
+
+
 def main():
+    if config.DEMO:
+        run_demo_mode()
+        return
+
     atexit.register(lambda: print(CURSOR_ON, end="", flush=True))
 
     def handle_sigint(*_):