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

effects_plugins/hud.py

@@ -0,0 +1,61 @@
+from engine.effects.performance import get_monitor
+from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
+
+
+class HudEffect(EffectPlugin):
+    name = "hud"
+    config = EffectConfig(enabled=True, intensity=1.0)
+
+    def process(self, buf: list[str], ctx: EffectContext) -> list[str]:
+        result = list(buf)
+        monitor = get_monitor()
+
+        fps = 0.0
+        frame_time = 0.0
+        if monitor:
+            stats = monitor.get_stats()
+            if stats:
+                fps = stats.fps
+                frame_time = stats.avg_frame_time_ms
+
+        w = ctx.terminal_width
+        h = ctx.terminal_height
+
+        effect_name = self.config.params.get("display_effect", "none")
+        effect_intensity = self.config.params.get("display_intensity", 0.0)
+
+        hud_lines = []
+        hud_lines.append(
+            f"\033[1;1H\033[38;5;46mMAINLINE DEMO\033[0m \033[38;5;245m|\033[0m \033[38;5;39mFPS: {fps:.1f}\033[0m \033[38;5;245m|\033[0m \033[38;5;208m{frame_time:.1f}ms\033[0m"
+        )
+
+        bar_width = 20
+        filled = int(bar_width * effect_intensity)
+        bar = (
+            "\033[38;5;82m"
+            + "█" * filled
+            + "\033[38;5;240m"
+            + "░" * (bar_width - filled)
+            + "\033[0m"
+        )
+        hud_lines.append(
+            f"\033[2;1H\033[38;5;45mEFFECT:\033[0m \033[1;38;5;227m{effect_name:12s}\033[0m \033[38;5;245m|\033[0m {bar} \033[38;5;245m|\033[0m \033[38;5;219m{effect_intensity * 100:.0f}%\033[0m"
+        )
+
+        from engine.effects import get_effect_chain
+
+        chain = get_effect_chain()
+        order = chain.get_order()
+        pipeline_str = ",".join(order) if order else "(none)"
+        hud_lines.append(f"\033[3;1H\033[38;5;44mPIPELINE:\033[0m {pipeline_str}")
+
+        for i, line in enumerate(hud_lines):
+            if i < len(result):
+                result[i] = line + result[i][len(line) :]
+            else:
+                result.append(line)
+
+        return result
+
+    def configure(self, config: EffectConfig) -> None:
+        self.config = config

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(*_):

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."""