feat(demo): add HUD effect, resize handling, and tests

- Add HUD effect plugin showing FPS, effect name, intensity bar, pipeline
- Add pygame window resize handling (VIDEORESIZE event)
- Move HUD to end of chain so it renders on top
- Fix monitor stats API (returns dict, not object)
- Add tests/test_hud.py for HUD effect verification

engine/app.py

@@ -376,7 +376,7 @@ def run_demo_mode():
 
     registry = get_registry()
     chain = get_effect_chain()
-    chain.set_order(["hud", "noise", "fade", "glitch", "firehose"])
+    chain.set_order(["noise", "fade", "glitch", "firehose", "hud"])
 
     monitor = PerformanceMonitor()
     set_monitor(monitor)
@@ -512,6 +512,13 @@ def run_demo_mode():
             result = chain.process(buf, ctx)
             display.show(result)
 
+            new_w, new_h = display.get_dimensions()
+            if new_w != w or new_h != h:
+                w, h = new_w, new_h
+                scroll_step_interval = calculate_scroll_step(config.SCROLL_DUR, h)
+                active = []
+                noise_cache = {}
+
             frame_number += 1
             time.sleep(1 / 60)