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.
