refactor: Rename click property to on_click (closes #139)

Breaking change: callback property standardized to on_* pattern.
- `drawable.click` → `drawable.on_click`

Updated all C++ bindings (8 files) and Python test usages.
Note: src/scripts changes tracked separately (in .gitignore).

This establishes the naming pattern for future callbacks:
on_click, on_enter, on_exit, on_move, on_key, etc.

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

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

tests/unit/WORKING_automation_test_example.py

@@ -68,7 +68,7 @@ frame.children.append(caption)
 def frame_clicked(x, y, button):
     print(f"Frame clicked at ({x}, {y}) with button {button}")
 
-frame.click = frame_clicked
+frame.on_click = frame_clicked
 
 print("Scene setup complete. Setting timer for automation tests...")