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 { }

SVSim.BattleNode/Protocol/RawBody.cs

@@ -0,0 +1,16 @@
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>
+/// Wraps a parsed-dictionary body for the inbound path. <see cref="MsgEnvelope.FromJson"/>
+/// returns this; <see cref="MsgEnvelope.ToJson"/> flattens <see cref="Entries"/> back to
+/// top-level keys when echoing.
+/// </summary>
+public sealed class RawBody : IMsgBody
+{
+    public Dictionary<string, object?> Entries { get; }
+
+    public RawBody(Dictionary<string, object?> entries)
+    {
+        Entries = entries;
+    }
+}