refactor(item): decouple items from block types

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.

src/CMakeLists.txt

@@ -105,4 +105,5 @@ target_sources(${PROJECT_NAME}
         gameplay/systems/wander_ai_system.cpp
         tools/json_utils.cpp
         gameplay/item_manager.cpp
+        gameplay/block_manager.cpp
 )