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

tests/test_pipeline.py

@@ -627,12 +627,12 @@ class TestStageAdapters:
         from engine.pipeline.adapters import CameraStage
         from engine.pipeline.core import PipelineContext
 
-        camera = Camera(mode=CameraMode.VERTICAL)
+        camera = Camera(mode=CameraMode.FEED)
         stage = CameraStage(camera, name="vertical")
         PipelineContext()
 
         assert "camera" in stage.capabilities
-        assert "source" in stage.dependencies  # Prefix matches any source
+        assert "render.output" in stage.dependencies  # Depends on rendered content
 
 
 class TestDataSourceStage: