fix(battle-node): MsgEnvelope rejects reserved Body keys + complete ReceiveNodeResultCode

ToJson now throws ArgumentException when a Body key collides with a reserved
envelope field (uri/viewerId/uuid/bid/try/cat/pubSeq/playSeq); FromJson reuses
the same shared ReservedEnvelopeKeys HashSet. ReceiveNodeResultCode expanded
from 9 to 31 codes to mirror the full enums.md catalog. Two regression tests
added for the collision guard and PascalCase uri serialization.

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

SVSim.UnitTests/BattleNode/Protocol/MsgEnvelopeTests.cs

@@ -63,4 +63,42 @@ public class MsgEnvelopeTests
         Assert.That(env.Uri, Is.EqualTo(NetworkBattleUri.PlayActions));
         Assert.That(env.Cat, Is.EqualTo(EmitCategory.Battle));
     }
+
+    [Test]
+    public void ToJson_BodyContainingReservedKey_Throws()
+    {
+        var env = new MsgEnvelope(
+            Uri: NetworkBattleUri.Loaded,
+            ViewerId: 1,
+            Uuid: "u",
+            Bid: null,
+            Try: 0,
+            Cat: EmitCategory.Battle,
+            PubSeq: null,
+            PlaySeq: null,
+            Body: new Dictionary<string, object?> { ["uri"] = "Injected" });
+
+        var ex = Assert.Throws<ArgumentException>(() => MsgEnvelope.ToJson(env));
+        Assert.That(ex!.Message, Does.Contain("uri"));
+    }
+
+    [Test]
+    public void ToJson_UriField_SerializesAsExactPascalCaseMemberName()
+    {
+        var env = new MsgEnvelope(
+            Uri: NetworkBattleUri.PlayActions,
+            ViewerId: 1,
+            Uuid: "u",
+            Bid: null,
+            Try: 0,
+            Cat: EmitCategory.Battle,
+            PubSeq: null,
+            PlaySeq: null,
+            Body: new Dictionary<string, object?>());
+
+        var json = MsgEnvelope.ToJson(env);
+
+        // Wire form must be PascalCase exactly — not "playActions", not "play_actions".
+        Assert.That(json, Does.Contain("\"uri\":\"PlayActions\""));
+    }
 }