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/pipeline/presets.py

@@ -45,7 +45,7 @@ class PipelinePreset:
     description: str = ""
     source: str = "headlines"
     display: str = "terminal"
-    camera: str = "vertical"
+    camera: str = "scroll"
     effects: list[str] = field(default_factory=list)
     border: bool = False
 
@@ -79,7 +79,7 @@ DEMO_PRESET = PipelinePreset(
     description="Demo mode with effect cycling and camera modes",
     source="headlines",
     display="pygame",
-    camera="vertical",
+    camera="scroll",
     effects=["noise", "fade", "glitch", "firehose", "hud"],
 )
 
@@ -88,7 +88,7 @@ POETRY_PRESET = PipelinePreset(
     description="Poetry feed with subtle effects",
     source="poetry",
     display="pygame",
-    camera="vertical",
+    camera="scroll",
     effects=["fade", "hud"],
 )
 
@@ -106,7 +106,7 @@ WEBSOCKET_PRESET = PipelinePreset(
     description="WebSocket display mode",
     source="headlines",
     display="websocket",
-    camera="vertical",
+    camera="scroll",
     effects=["noise", "fade", "glitch", "hud"],
 )
 
@@ -115,7 +115,7 @@ SIXEL_PRESET = PipelinePreset(
     description="Sixel graphics display mode",
     source="headlines",
     display="sixel",
-    camera="vertical",
+    camera="scroll",
     effects=["noise", "fade", "glitch", "hud"],
 )
 
@@ -124,7 +124,7 @@ FIREHOSE_PRESET = PipelinePreset(
     description="High-speed firehose mode",
     source="headlines",
     display="pygame",
-    camera="vertical",
+    camera="scroll",
     effects=["noise", "fade", "glitch", "firehose", "hud"],
 )