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/UIDrawable.h

@@ -36,12 +36,29 @@ public:
     virtual void render(sf::Vector2f, sf::RenderTarget&) = 0;
     virtual PyObjectsEnum derived_type() = 0;
 
-    // Mouse input handling - callable object, methods to find event's destination
+    // Mouse input handling - callable objects for click, enter, exit, move events
     std::unique_ptr<PyClickCallable> click_callable;
+    std::unique_ptr<PyClickCallable> on_enter_callable;   // #140
+    std::unique_ptr<PyClickCallable> on_exit_callable;    // #140
+    std::unique_ptr<PyClickCallable> on_move_callable;    // #141
+
     virtual UIDrawable* click_at(sf::Vector2f point) = 0;
     void click_register(PyObject*);
     void click_unregister();
 
+    // #140 - Mouse enter/exit callbacks
+    void on_enter_register(PyObject*);
+    void on_enter_unregister();
+    void on_exit_register(PyObject*);
+    void on_exit_unregister();
+
+    // #141 - Mouse move callback
+    void on_move_register(PyObject*);
+    void on_move_unregister();
+
+    // #140 - Hovered state (set by GameEngine)
+    bool hovered = false;
+
     UIDrawable();
     virtual ~UIDrawable();
     
@@ -55,6 +72,15 @@ public:
 
     static PyObject* get_click(PyObject* self, void* closure);
     static int set_click(PyObject* self, PyObject* value, void* closure);
+    // #140 - Python API for on_enter/on_exit callbacks
+    static PyObject* get_on_enter(PyObject* self, void* closure);
+    static int set_on_enter(PyObject* self, PyObject* value, void* closure);
+    static PyObject* get_on_exit(PyObject* self, void* closure);
+    static int set_on_exit(PyObject* self, PyObject* value, void* closure);
+    static PyObject* get_hovered(PyObject* self, void* closure);
+    // #141 - Python API for on_move callback
+    static PyObject* get_on_move(PyObject* self, void* closure);
+    static int set_on_move(PyObject* self, PyObject* value, void* closure);
     static PyObject* get_int(PyObject* self, void* closure);
     static int set_int(PyObject* self, PyObject* value, void* closure);
     static PyObject* get_name(PyObject* self, void* closure);