fix(battle-node): SocketIoFrame disposal safety + escaping + empty-args encoding

- Wrap all JsonDocument.Parse calls in using blocks and Clone() each
  retained JsonElement to eliminate UAF hazard after GC.
- Use JsonSerializer.Serialize with UnsafeRelaxedJsonEscaping so event
  names with " or \ produce \" / \ rather than " / plain \;
  avoids malformed JSON on Encode().
- Guard the [ ] block in Encode() behind EventName-or-args check so
  Connect/Disconnect packets round-trip as bare "0"/"1" not "0[]".
- Add three regression tests: Connect no-bracket, Event round-trip,
  special-char event name escaping.

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

SVSim.UnitTests/BattleNode/Wire/SocketIoFrameTests.cs

@@ -85,4 +85,34 @@ public class SocketIoFrameTests
         var header = SocketIoFrame.Parse("51-[\"msg\",{\"_placeholder\":true,\"num\":0}]");
         Assert.Throws<ArgumentException>(() => header.WithAttachments(Array.Empty<byte[]>()));
     }
+
+    [Test]
+    public void Encode_ConnectPacket_HasNoBracketedArgs()
+    {
+        // Regression for an earlier bug where Encode always emitted "[]".
+        var frame = SocketIoFrame.Parse("0");
+        var (text, bins) = frame.Encode();
+        Assert.That(text, Is.EqualTo("0"));
+        Assert.That(bins, Is.Empty);
+    }
+
+    [Test]
+    public void ParseThenEncode_EventWithAck_RoundTripsByteForByte()
+    {
+        const string wire = "27[\"msg\",42]";
+        var frame = SocketIoFrame.Parse(wire);
+        var (text, _) = frame.Encode();
+        Assert.That(text, Is.EqualTo(wire));
+    }
+
+    [Test]
+    public void Encode_EventNameWithSpecialChars_IsJsonEscaped()
+    {
+        var frame = SocketIoFrame.BinaryEventWithAttachments(
+            eventName: "weird \"name\" with \\ backslash",
+            attachments: new[] { new byte[] { 0x00 } });
+        var (text, _) = frame.Encode();
+        // The event name must be JSON-escaped: each " becomes \", and the literal \ becomes \\.
+        Assert.That(text, Does.Contain("\"weird \\\"name\\\" with \\\\ backslash\""));
+    }
 }