feat(battle-node): EngineIoFrame parse/encode for EIO3 packets

SVSim.BattleNode/Wire/EngineIoFrame.cs

@@ -0,0 +1,21 @@
+namespace SVSim.BattleNode.Wire;
+
+/// <summary>
+/// Engine.IO v3 packet in WebSocket transport mode. Wire form: <c>&lt;digit&gt;&lt;payload&gt;</c>.
+/// </summary>
+public sealed record EngineIoFrame(EngineIoPacketType Type, string Payload)
+{
+    public static EngineIoFrame Parse(string raw)
+    {
+        if (string.IsNullOrEmpty(raw))
+            throw new ArgumentException("Empty EIO frame", nameof(raw));
+        var typeChar = raw[0];
+        if (typeChar < '0' || typeChar > '6')
+            throw new ArgumentException($"Invalid EIO type char '{typeChar}'", nameof(raw));
+        var type = (EngineIoPacketType)(typeChar - '0');
+        var payload = raw.Length > 1 ? raw.Substring(1) : string.Empty;
+        return new EngineIoFrame(type, payload);
+    }
+
+    public string Encode() => $"{(int)Type}{Payload}";
+}