Add pig spawn egg asset and texture, parse spawn egg item kind and creature property, and spawn the configured entity when used on an empty block. Also fix item texture loading to use actual image dimensions and update selected item UI sizing.
The item system now supports an ItemKind and property, allowing item JSON to declare a type instead of assuming every item is a block. BlockManager was moved to its own header, redesigned around concurrent hash maps, and exposes id_from_name(). Texture and UI code now key items by ItemID, while block placement resolves the block type from the item registry.
Implement ItemManager to load and query item definitions from assets/item JSON files. Add ItemData struct, item asset metadata, CMake source registration, and initialize the manager during app startup. Also add AGENTS.md repository guidelines.
