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/server/profile.go

@@ -0,0 +1,21 @@
+package server
+
+import (
+	"crypto/md5"
+)
+
+// Profile is an authenticated (or, in offline mode, derived) player identity.
+type Profile struct {
+	UUID [16]byte
+	Name string
+}
+
+// OfflineUUID derives the stable offline-mode UUID for a username, matching
+// vanilla's java.util.UUID.nameUUIDFromBytes("OfflinePlayer:" + name): an
+// MD5-based (version 3) UUID with the IETF variant bits set.
+func OfflineUUID(name string) [16]byte {
+	sum := md5.Sum([]byte("OfflinePlayer:" + name))
+	sum[6] = (sum[6] & 0x0f) | 0x30 // version 3
+	sum[8] = (sum[8] & 0x3f) | 0x80 // IETF variant
+	return sum
+}