feat(battle-node): add IMsgBody marker + RawBody inbound wrapper

SVSim.BattleNode/Protocol/IMsgBody.cs

@@ -0,0 +1,10 @@
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>
+/// Marker for every type that can appear as <see cref="MsgEnvelope.Body"/>.
+/// Implementers fall into two camps: typed records used on the outbound path
+/// (one per scripted frame shape) and <see cref="RawBody"/> used on the inbound
+/// path. The marker exists so the envelope can carry either without falling
+/// back to <c>object</c>.
+/// </summary>
+public interface IMsgBody { }