Fix #221: Add grid_pos and grid_size properties for Grid children

UIDrawables placed in a Grid's children collection now have:
- grid_pos: Position in tile coordinates (get/set)
- grid_size: Size in tile coordinates (get/set)

Raises RuntimeError if accessed when parent is not a Grid.
UIGrid only gets grid_pos (grid_size conflicts with existing property).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/UIGrid.h

@@ -135,6 +135,9 @@ public:
     // #142 - Cell coordinate conversion (screen pos -> cell coords)
     std::optional<sf::Vector2i> screenToCell(sf::Vector2f screen_pos) const;
 
+    // #221 - Get effective cell size (texture size * zoom)
+    sf::Vector2f getEffectiveCellSize() const;
+
     // #142 - Update cell hover state (called from PyScene)
     void updateCellHover(sf::Vector2f mousepos);