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/display/backends/pygame.py

@@ -36,6 +36,7 @@ class PygameDisplay:
         self._pygame = None
         self._screen = None
         self._font = None
+        self._resized = False
 
     def _get_font_path(self) -> str | None:
         """Get font path for rendering."""
@@ -110,7 +111,10 @@ class PygameDisplay:
         pygame.init()
         pygame.display.set_caption("Mainline")
 
-        self._screen = pygame.display.set_mode((self.window_width, self.window_height))
+        self._screen = pygame.display.set_mode(
+            (self.window_width, self.window_height),
+            pygame.RESIZABLE,
+        )
         self._pygame = pygame
         PygameDisplay._pygame_initialized = True
 
@@ -136,6 +140,12 @@ class PygameDisplay:
         for event in self._pygame.event.get():
             if event.type == self._pygame.QUIT:
                 sys.exit(0)
+            elif event.type == self._pygame.VIDEORESIZE:
+                self.window_width = event.w
+                self.window_height = event.h
+                self.width = max(1, self.window_width // self.cell_width)
+                self.height = max(1, self.window_height // self.cell_height)
+                self._resized = True
 
         self._screen.fill((0, 0, 0))
 
@@ -175,6 +185,16 @@ class PygameDisplay:
             self._screen.fill((0, 0, 0))
             self._pygame.display.flip()
 
+    def get_dimensions(self) -> tuple[int, int]:
+        """Get current terminal dimensions based on window size.
+
+        Returns:
+            (width, height) in character cells
+        """
+        if self._resized:
+            self._resized = False
+        return self.width, self.height
+
     def cleanup(self, quit_pygame: bool = True) -> None:
         """Cleanup display resources.