#303 Add Cow entity implementation and tests

[Anonymous]

Originally created by: starwarslevi

This adds the Cow entity and wires it into the entity registry/export path. Cow now has vanilla-style behavior implemented (milking, sounds, breeding variant inheritance, spawn variant + sound-variant selection, and NBT save/load for cow-specific data). It also adds focused Cow tests for behavior and persistence parity

https://github.com/user-attachments/assets/8e84ab62-6b33-48ff-8d34-ebe3486f8122