feat(pipeline): add Canvas and FontStage for rendering

- Add Canvas class for 2D surface management
- Add CanvasStage for pipeline integration
- Add FontStage as Transform for font rendering
- Update Camera with x, y, w, h, zoom and guardrails
- Add get_dimensions() to Display protocol

engine/display/__init__.py

@@ -67,6 +67,18 @@ class Display(Protocol):
         """Shutdown display."""
         ...
 
+    def get_dimensions(self) -> tuple[int, int]:
+        """Get current terminal dimensions.
+
+        Returns:
+            (width, height) in character cells
+
+        This method is called after show() to check if the display
+        was resized. The main loop should compare this to the current
+        viewport dimensions and update accordingly.
+        """
+        ...
+
 
 class DisplayRegistry:
     """Registry for display backends with auto-discovery."""