GroundZeroCore RP v19 validation

Shroom progress GUI texture:
- Core now uses gzItem("shrooms", 1) for the Drug Producer Progress Shrooms icon.
- This uses brown_mushroom custom_model_data 9104.
- Resource pack already contains:
  assets/minecraft/models/item/brown_mushroom.json override 9104 -> minecraft:item/gz/shrooms
  assets/minecraft/models/item/gz/shrooms.json
  assets/minecraft/textures/item/gz/shrooms.png

No existing textures were removed.
