Initial commit: RegionIO Minecraft server core (26.1.2/protocol 775)

Vanilla-faithful overworld generator (final_density + multi-noise biomes),
full connection lifecycle (status/login/configuration/play), chunk streaming,
creative block editing, and the protocol/nbt/registry infrastructure.

internal/world/items.go

@@ -0,0 +1,38 @@
+package world
+
+import (
+	_ "embed"
+	"encoding/json"
+	"fmt"
+	"strconv"
+)
+
+//go:embed item_blocks.json
+var itemBlocksJSON []byte
+
+// itemToBlock maps an item's network ID to the default block state placed when
+// that item is used. Built from the item registry crossed with block defaults
+// (items whose name is a block). Items with no block (tools, food) are absent.
+var itemToBlock map[int32]uint16
+
+func init() {
+	raw := make(map[string]int)
+	if err := json.Unmarshal(itemBlocksJSON, &raw); err != nil {
+		panic(fmt.Sprintf("world: parsing item_blocks.json: %v", err))
+	}
+	itemToBlock = make(map[int32]uint16, len(raw))
+	for k, v := range raw {
+		id, err := strconv.Atoi(k)
+		if err != nil {
+			panic(fmt.Sprintf("world: bad item id %q: %v", k, err))
+		}
+		itemToBlock[int32(id)] = uint16(v)
+	}
+}
+
+// ItemToBlock returns the block state placed by an item, and whether the item
+// is a placeable block.
+func ItemToBlock(itemID int32) (uint16, bool) {
+	s, ok := itemToBlock[itemID]
+	return s, ok
+}