feat: Animation property locking prevents conflicting animations (closes #120)

Add AnimationConflictMode enum with three modes:
- REPLACE (default): Complete existing animation and start new one
- QUEUE: Wait for existing animation to complete before starting
- ERROR: Raise RuntimeError if property is already being animated

Changes:
- AnimationManager now tracks property locks per (target, property) pair
- Animation.start() accepts optional conflict_mode parameter
- Queued animations start automatically when property becomes free
- Updated type stubs with ConflictMode type alias

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

src/PyAnimation.h

@@ -25,7 +25,7 @@ public:
     static PyObject* get_is_delta(PyAnimationObject* self, void* closure);
     
     // Methods
-    static PyObject* start(PyAnimationObject* self, PyObject* args);
+    static PyObject* start(PyAnimationObject* self, PyObject* args, PyObject* kwds);
     static PyObject* update(PyAnimationObject* self, PyObject* args);
     static PyObject* get_current_value(PyAnimationObject* self, PyObject* args);
     static PyObject* complete(PyAnimationObject* self, PyObject* args);