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

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

SVSim.UnitTests/BattleNode/Protocol/MsgEnvelopeTests.cs

@@ -0,0 +1,66 @@
+using System.Text.Json;
+using NUnit.Framework;
+using SVSim.BattleNode.Protocol;
+
+namespace SVSim.UnitTests.BattleNode.Protocol;
+
+[TestFixture]
+public class MsgEnvelopeTests
+{
+    [Test]
+    public void Roundtrip_PreservesEnvelopeAndBody()
+    {
+        var env = new MsgEnvelope(
+            Uri: NetworkBattleUri.InitNetwork,
+            ViewerId: 906243102,
+            Uuid: "udid-1234",
+            Bid: "597830888107",
+            Try: 0,
+            Cat: EmitCategory.General,
+            PubSeq: null,
+            PlaySeq: null,
+            Body: new Dictionary<string, object?> { ["foo"] = 42 });
+
+        var json = MsgEnvelope.ToJson(env);
+        var back = MsgEnvelope.FromJson(json);
+
+        Assert.That(back.Uri, Is.EqualTo(NetworkBattleUri.InitNetwork));
+        Assert.That(back.ViewerId, Is.EqualTo(906243102));
+        Assert.That(back.Uuid, Is.EqualTo("udid-1234"));
+        Assert.That(back.Bid, Is.EqualTo("597830888107"));
+        Assert.That(back.Cat, Is.EqualTo(EmitCategory.General));
+        Assert.That(back.Body["foo"], Is.EqualTo(42L));   // JsonElement → int64
+    }
+
+    [Test]
+    public void ToJson_OmitsNullEnvelopeFields()
+    {
+        var env = new MsgEnvelope(
+            Uri: NetworkBattleUri.Ready,
+            ViewerId: 1,
+            Uuid: "u",
+            Bid: null,
+            Try: 0,
+            Cat: EmitCategory.Battle,
+            PubSeq: null,
+            PlaySeq: 5,
+            Body: new Dictionary<string, object?>());
+
+        var json = MsgEnvelope.ToJson(env);
+
+        Assert.That(json, Does.Not.Contain("\"bid\""));
+        Assert.That(json, Does.Not.Contain("\"pubSeq\""));
+        Assert.That(json, Does.Contain("\"playSeq\":5"));
+    }
+
+    [Test]
+    public void FromJson_DispatchesUriToEnum()
+    {
+        const string json = "{\"uri\":\"PlayActions\",\"viewerId\":1,\"uuid\":\"u\",\"try\":0,\"cat\":1}";
+
+        var env = MsgEnvelope.FromJson(json);
+
+        Assert.That(env.Uri, Is.EqualTo(NetworkBattleUri.PlayActions));
+        Assert.That(env.Cat, Is.EqualTo(EmitCategory.Battle));
+    }
+}