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>

tests/demo_animation_callback_usage.py

@@ -0,0 +1,81 @@
+#!/usr/bin/env python3
+"""
+Demonstration of animation callbacks solving race conditions.
+Shows how callbacks enable direct causality for game state changes.
+"""
+
+import mcrfpy
+
+# Game state
+player_moving = False
+move_queue = []
+
+def movement_complete(anim, target):
+    """Called when player movement animation completes"""
+    global player_moving, move_queue
+    
+    print("Movement animation completed!")
+    player_moving = False
+    
+    # Process next move if queued
+    if move_queue:
+        next_pos = move_queue.pop(0)
+        move_player_to(next_pos)
+    else:
+        print("Player is now idle and ready for input")
+
+def move_player_to(new_pos):
+    """Move player with animation and proper state management"""
+    global player_moving
+    
+    if player_moving:
+        print(f"Queueing move to {new_pos}")
+        move_queue.append(new_pos)
+        return
+    
+    player_moving = True
+    print(f"Moving player to {new_pos}")
+    
+    # Get player entity (placeholder for demo)
+    ui = mcrfpy.sceneUI("game")
+    player = ui[0]  # Assume first element is player
+    
+    # Animate movement with callback
+    x, y = new_pos
+    anim_x = mcrfpy.Animation("x", float(x), 0.5, "easeInOutQuad", callback=movement_complete)
+    anim_y = mcrfpy.Animation("y", float(y), 0.5, "easeInOutQuad")
+    
+    anim_x.start(player)
+    anim_y.start(player)
+
+def setup_demo():
+    """Set up the demo scene"""
+    # Create scene
+    mcrfpy.createScene("game")
+    mcrfpy.setScene("game")
+    
+    # Create player sprite
+    player = mcrfpy.Frame((100, 100), (32, 32), fill_color=(0, 255, 0))
+    ui = mcrfpy.sceneUI("game")
+    ui.append(player)
+    
+    print("Demo: Animation callbacks for movement queue")
+    print("=" * 40)
+    
+    # Simulate rapid movement commands
+    mcrfpy.setTimer("move1", lambda r: move_player_to((200, 100)), 100)
+    mcrfpy.setTimer("move2", lambda r: move_player_to((200, 200)), 200)  # Will be queued
+    mcrfpy.setTimer("move3", lambda r: move_player_to((100, 200)), 300)  # Will be queued
+    
+    # Exit after demo
+    mcrfpy.setTimer("exit", lambda r: exit_demo(), 3000)
+
+def exit_demo():
+    """Exit the demo"""
+    print("\nDemo completed successfully!")
+    print("Callbacks ensure proper movement sequencing without race conditions")
+    import sys
+    sys.exit(0)
+
+# Run the demo
+setup_demo()