Place mob icon PNGs in this folder.

Naming:
- Base icon: <entity_id>.png
  Example: zombie.png, skeleton.png, enderman.png
- Variant icon: <entity_id>_<variant>.png
  Example: frog_temperate.png, rabbit_toast.png
- Fallback icon: _fallback.png

The editor NPC list resolves icons in this order:
1) <entity_id>_<variant>.png
2) <entity_id>.png
3) _fallback.png

Texture path namespace:
- npcstudio:textures/gui/mob_icons/<name>.png
