feat(battle-node): NetworkBattleUri / EmitCategory enums and MsgEnvelope record

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.BattleNode/Protocol/EmitCategory.cs

@@ -0,0 +1,11 @@
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>The "cat" field on the msg envelope.</summary>
+public enum EmitCategory
+{
+    Battle = 1,
+    Matching = 2,
+    Room = 3,
+    Watch = 11,
+    General = 99,
+}