Add animation completion callbacks (#119)

Implement callbacks that fire when animations complete, enabling direct
causality between animation end and game state changes. This eliminates
race conditions from parallel timer workarounds.

- Add optional callback parameter to Animation constructor
- Callbacks execute synchronously when animation completes
- Proper Python reference counting with GIL safety
- Callbacks receive (anim, target) parameters (currently None)
- Exception handling prevents crashes from Python errors

Example usage:
```python
def on_complete(anim, target):
    player_moving = False

anim = mcrfpy.Animation("x", 300.0, 1.0, "easeOut", callback=on_complete)
anim.start(player)
```

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

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

src/PyAnimation.cpp

@@ -18,19 +18,31 @@ PyObject* PyAnimation::create(PyTypeObject* type, PyObject* args, PyObject* kwds
 }
 
 int PyAnimation::init(PyAnimationObject* self, PyObject* args, PyObject* kwds) {
-    static const char* keywords[] = {"property", "target", "duration", "easing", "delta", nullptr};
+    static const char* keywords[] = {"property", "target", "duration", "easing", "delta", "callback", nullptr};
     
     const char* property_name;
     PyObject* target_value;
     float duration;
     const char* easing_name = "linear";
     int delta = 0;
+    PyObject* callback = nullptr;
     
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "sOf|sp", const_cast<char**>(keywords),
-                                      &property_name, &target_value, &duration, &easing_name, &delta)) {
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "sOf|spO", const_cast<char**>(keywords),
+                                      &property_name, &target_value, &duration, &easing_name, &delta, &callback)) {
         return -1;
     }
     
+    // Validate callback is callable if provided
+    if (callback && callback != Py_None && !PyCallable_Check(callback)) {
+        PyErr_SetString(PyExc_TypeError, "callback must be callable");
+        return -1;
+    }
+    
+    // Convert None to nullptr for C++
+    if (callback == Py_None) {
+        callback = nullptr;
+    }
+    
     // Convert Python target value to AnimationValue
     AnimationValue animValue;
     
@@ -90,7 +102,7 @@ int PyAnimation::init(PyAnimationObject* self, PyObject* args, PyObject* kwds) {
     EasingFunction easingFunc = EasingFunctions::getByName(easing_name);
     
     // Create the Animation
-    self->data = std::make_shared<Animation>(property_name, animValue, duration, easingFunc, delta != 0);
+    self->data = std::make_shared<Animation>(property_name, animValue, duration, easingFunc, delta != 0, callback);
     
     return 0;
 }