feat(core): add Camera abstraction for viewport scrolling

- Add Camera class with modes: vertical, horizontal, omni, floating
- Refactor scroll.py and demo to use Camera abstraction
- Add vis_offset for horizontal scrolling support
- Add camera_x to EffectContext for effects
- Add pygame window resize handling
- Add HUD effect plugin for demo mode
- Add --demo flag to run demo mode
- Add tests for Camera and vis_offset

engine/effects/types.py

@@ -29,10 +29,11 @@ class EffectContext:
     terminal_height: int
     scroll_cam: int
     ticker_height: int
-    mic_excess: float
-    grad_offset: float
-    frame_number: int
-    has_message: bool
+    camera_x: int = 0
+    mic_excess: float = 0.0
+    grad_offset: float = 0.0
+    frame_number: int = 0
+    has_message: bool = False
     items: list = field(default_factory=list)