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_camera.py

@@ -1,19 +1,18 @@
-
 from engine.camera import Camera, CameraMode
 
 
 def test_camera_vertical_default():
     """Test default vertical camera."""
     cam = Camera()
-    assert cam.mode == CameraMode.VERTICAL
+    assert cam.mode == CameraMode.FEED
     assert cam.x == 0
     assert cam.y == 0
 
 
 def test_camera_vertical_factory():
     """Test vertical factory method."""
-    cam = Camera.vertical(speed=2.0)
-    assert cam.mode == CameraMode.VERTICAL
+    cam = Camera.feed(speed=2.0)
+    assert cam.mode == CameraMode.FEED
     assert cam.speed == 2.0
 
 

tests/test_performance_regression.py

@@ -75,8 +75,8 @@ class TestViewportFilterPerformance:
 
         With 1438 items and 24-line viewport:
         - Without filter: FontStage renders all 1438 items
-        - With filter: FontStage renders ~5 items
-        - Expected improvement: 1438 / 5 ≈ 288x
+        - With filter: FontStage renders ~3 items (layout-based)
+        - Expected improvement: 1438 / 3 ≈ 479x
         """
         test_items = [
             SourceItem(f"Headline {i}", "source", str(i)) for i in range(1438)
@@ -89,10 +89,10 @@ class TestViewportFilterPerformance:
         filtered = stage.process(test_items, ctx)
         improvement_factor = len(test_items) / len(filtered)
 
-        # Verify we get expected 288x improvement
-        assert 250 < improvement_factor < 300
-        # Verify filtered count is reasonable
-        assert 4 <= len(filtered) <= 6
+        # Verify we get expected ~479x improvement (better than old ~288x)
+        assert 400 < improvement_factor < 600
+        # Verify filtered count is reasonable (layout-based is more precise)
+        assert 2 <= len(filtered) <= 5
 
 
 class TestPipelinePerformanceWithRealData:

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:

tests/test_viewport_filter_performance.py

@@ -5,7 +5,6 @@ of items processed by FontStage, preventing the 10+ second hangs observed with
 large headline sources.
 """
 
-
 from engine.data_sources.sources import SourceItem
 from engine.pipeline.adapters import ViewportFilterStage
 from engine.pipeline.core import PipelineContext
@@ -97,7 +96,8 @@ class TestViewportFilterStage:
         # With 1438 items and 24-line viewport:
         # - Without filter: FontStage renders all 1438 items
         # - With filter: FontStage renders only ~5 items
-        # - Improvement: 1438 / 5 = 287.6x fewer items to render
+        # - Improvement: 1438 / 3 = ~479x fewer items to render
+        #   (layout-based filtering is more precise than old estimate)
 
         test_items = [
             SourceItem(f"Headline {i}", "source", str(i)) for i in range(1438)
@@ -110,10 +110,10 @@ class TestViewportFilterStage:
         filtered = stage.process(test_items, ctx)
         improvement_factor = len(test_items) / len(filtered)
 
-        # Verify we get at least 200x improvement
-        assert improvement_factor > 200
-        # Verify we get the expected ~288x improvement
-        assert 250 < improvement_factor < 300
+        # Verify we get at least 400x improvement (better than old ~288x)
+        assert improvement_factor > 400
+        # Verify we get the expected ~479x improvement
+        assert 400 < improvement_factor < 600
 
 
 class TestViewportFilterIntegration: