feat(battle-node): MsgPayloadCodec encodes/decodes msgpack↔envelope chain

SVSim.BattleNode/Protocol/MsgPayloadCodec.cs

@@ -0,0 +1,26 @@
+using MessagePack;
+using SVSim.BattleNode.Wire;
+
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>
+/// Full chain between an envelope and the bytes that ride as a SocketIO binary attachment.
+/// Inbound:  bytes → msgpack-string → NodeCrypto.Decrypt → JSON → MsgEnvelope
+/// Outbound: MsgEnvelope → JSON → NodeCrypto.Encrypt → msgpack-bytes
+/// </summary>
+public static class MsgPayloadCodec
+{
+    public static MsgEnvelope Decode(byte[] msgpackBytes)
+    {
+        var encryptedString = MessagePackSerializer.Deserialize<string>(msgpackBytes);
+        var json = NodeCrypto.DecryptForNode(encryptedString);
+        return MsgEnvelope.FromJson(json);
+    }
+
+    public static byte[] Encode(MsgEnvelope envelope, string key)
+    {
+        var json = MsgEnvelope.ToJson(envelope);
+        var encryptedString = NodeCrypto.EncryptForNode(json, key);
+        return MessagePackSerializer.Serialize(encryptedString);
+    }
+}