ambient occlusion issues

While the slabs added by the mod have AO correctly applied to them, snow_on_top (and other on_top block variants too, I assume) do not.

<img width="1920" height="1080" alt="Image" src="https://github.com/user-attachments/assets/41a6c0cf-6b41-4f64-813a-62af4796fadb" />