Mark SetHeldItem with item object as const ptr.

src/Battling/Models/Creature.hpp

@@ -87,7 +87,7 @@ namespace CreatureLib::Battling {
         }
         inline const Library::Item* GetHeldItem() const { return _heldItem; }
         void SetHeldItem(const std::string& itemName);
-        inline void SetHeldItem(Library::Item* item) { _heldItem = item; };
+        inline void SetHeldItem(const Library::Item* item) { _heldItem = item; };
 
         inline uint32_t GetCurrentHealth() const { return _currentHealth; }