feat: Implement comprehensive mouse event system

Implements multiple mouse event improvements for UI elements:

- Mouse enter/exit events (#140): on_enter, on_exit callbacks and
  hovered property for all UIDrawable types (Frame, Caption, Sprite, Grid)
- Headless click events (#111): Track simulated mouse position for
  automation testing in headless mode
- Mouse move events (#141): on_move callback fires continuously while
  mouse is within element bounds
- Grid cell events (#142): on_cell_enter, on_cell_exit, on_cell_click
  callbacks with cell coordinates (x, y), plus hovered_cell property

Includes comprehensive tests for all new functionality.

Closes #140, closes #111, closes #141, closes #142

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/PyScene.h

@@ -14,7 +14,8 @@ public:
     void render() override final;
 
     void do_mouse_input(std::string, std::string);
-    
+    void do_mouse_hover(int x, int y);  // #140 - Mouse enter/exit tracking
+
     // Dirty flag for z_index sorting optimization
     bool ui_elements_need_sort = true;
 };