strom: add 1.12.2 protocol support and legacy login/status handlers

- Add minecraft-data for 1.12.2 (protocol 340)
- Generate proto_generated/v1_12_2
- Add v1_12_2 legacy handlers to client.Login (OnDisconnect,
  OnCompress, OnSuccess) and client.Status (OnStatus, OnPong)
- Same pattern as v1_8: RegisterUntilLatest only registers
  upward, so legacy versions need explicit RegisterCritical handlers

mc/proto_generator/main.go

@@ -429,7 +429,7 @@ func GenerateVersions(versions []string) (err error) {
 }
 
 func main() {
-	err := GenerateVersions([]string{"1.8", "1.21.8", "1.21.9", "1.21.11", "26.1", "26.2"})
+	err := GenerateVersions([]string{"1.8", "1.12.2", "1.21.8", "1.21.9", "1.21.11", "26.1", "26.2"})
 	if err != nil {
 		panic(err)
 	}