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/data/minecraft-data/pc/1.12.2/version.json

@@ -0,0 +1,5 @@
+{
+  "minecraftVersion": "1.12.2",
+  "version":340,
+  "majorVersion": "1.12"
+}