feat(battle-node): add TurnEndBody record for opponent turn-end push

Mirrors OpponentTurnStartBody — JsonPropertyName-cased turnState +
resultCode (default 1). First piece of the scripted opponent turn-end
loop; nothing references it yet.

SVSim.BattleNode/Protocol/Bodies/TurnEndBody.cs

@@ -0,0 +1,7 @@
+using System.Text.Json.Serialization;
+
+namespace SVSim.BattleNode.Protocol.Bodies;
+
+public sealed record TurnEndBody(
+    [property: JsonPropertyName("turnState")] int TurnState,
+    [property: JsonPropertyName("resultCode")] int ResultCode = 1) : IMsgBody;