feat: Implement scrolling camera with layout-aware filtering

- Rename VERTICAL camera mode to FEED (rapid single-item view)
- Add SCROLL camera mode with float accumulation for smooth movie-credits style scrolling
- Add estimate_block_height() for cheap layout calculation without full rendering
- Replace ViewportFilterStage with layout-aware filtering that tracks camera position
- Add render caching to FontStage to avoid re-rendering items
- Fix CameraStage to use global canvas height for scrolling bounds
- Add horizontal padding in Camera.apply() to prevent ghosting
- Add get_dimensions() to MultiDisplay for proper viewport sizing
- Fix PygameDisplay to auto-detect viewport from window size
- Update presets to use scroll camera with appropriate speeds

engine/display/backends/pygame.py

@@ -122,6 +122,10 @@ class PygameDisplay:
         self._pygame = pygame
         PygameDisplay._pygame_initialized = True
 
+        # Calculate character dimensions from actual window size
+        self.width = max(1, self.window_width // self.cell_width)
+        self.height = max(1, self.window_height // self.cell_height)
+
         font_path = self._get_font_path()
         if font_path:
             try: