refactor(battle-node): switch MsgEnvelope.Body to IMsgBody, migrate all sites

SVSim.BattleNode/Reliability/Gungnir.cs

@@ -3,7 +3,10 @@ namespace SVSim.BattleNode.Reliability;
 /// <summary>
 /// Body builders for the alive channel. The timer/loop that drives 5s emits lives on
 /// BattleSession; this class is just the pure body-shape factory.
-/// v1 always reports scs/ocs=ONLINE — real disconnect detection is deferred.
+/// v1 always reports scs/ocs=ONLINE — real disconnect detection is deferred. The push
+/// body itself is constructed inline in BattleSession.HandleAliveEventAsync using
+/// AlivePushBody; only the emit body (sent by us TO the client on the alive channel,
+/// currently unused in v1) remains here.
 /// </summary>
 public static class Gungnir
 {
@@ -14,10 +17,4 @@ public static class Gungnir
         ["currentSeq"] = tracker.HighWaterMark,
         // actionSeq omitted in v1 — no turn-transition flag yet.
     };
-
-    public static Dictionary<string, object?> BuildAlivePushBody() => new()
-    {
-        ["scs"] = "ONLINE",
-        ["ocs"] = "ONLINE",
-    };
 }