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.

assets/item/dirt.json

@@ -2,5 +2,6 @@
     "id": 2,
     "name": "dirt",
     "description": "",
-    "texture": "texture/item/block/dirt.png"
-}
+    "texture": "texture/item/block/dirt.png",
+    "type": "block"
+}