Baku Nightmare Mobs 3D Update

What changed:
- Added custom 3D geometry for Shadow Stalker and Cave Screamer.
- Replaced flat image skins with UV-friendly solid textures to avoid holes.
- Added animated head/jaw expressions when targeting the player.
- Improved spawn egg textures.
- Kept random ambient horror sounds and natural dark spawning.

Commands:
/summon baku:shadow_stalker
/summon baku:cave_screamer
