Weapon slots, slot bg icons, weapon item types, BHS fix
- Add Staff, BHS item classes with icons
- Add weapon type stubs: Axe, Bow, Hammer, Fireball, EntropyVortex, HarmonicCalm
- Replace generic weapon slot array with typed weapon slots in InvUi
- Add bg icon field to BaseSlot::Ctor; show bg when slot is empty
- Add engineering version of background icon textures for all equipment and weapon slot types
- Add BHS and Staff test items to player inventory
- Fix BhsProjectile: remove projectile on non-mob collision
- Fix BhsProjectile: skip loot nodes in collision detection
- Fix varying.def.sc: i_data4 duplicate TEXCOORD5 → TEXCOORD3