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