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/air.json

@@ -1,5 +1,6 @@
 {
     "id": 0,
     "name": "air",
-    "description": ""
+    "description": "",
+    "type": "block"
 }