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}";
+}

SVSim.BattleNode/Wire/EngineIoHandshake.cs

@@ -0,0 +1,22 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace SVSim.BattleNode.Wire;
+
+/// <summary>
+/// Payload of an EIO3 Open packet. Sent by the server to the client immediately after the WS upgrade.
+/// </summary>
+public sealed record EngineIoHandshake(
+    [property: JsonPropertyName("sid")] string Sid,
+    [property: JsonPropertyName("upgrades")] string[] Upgrades,
+    [property: JsonPropertyName("pingInterval")] int PingInterval,
+    [property: JsonPropertyName("pingTimeout")] int PingTimeout)
+{
+    // Wire-key casing here is bare camelCase — NOT EmulatedEntrypoint's snake_case policy.
+    private static readonly JsonSerializerOptions Options = new()
+    {
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+    };
+
+    public string ToJson() => JsonSerializer.Serialize(this, Options);
+}

SVSim.BattleNode/Wire/EngineIoPacketType.cs

@@ -0,0 +1,12 @@
+namespace SVSim.BattleNode.Wire;
+
+public enum EngineIoPacketType
+{
+    Open = 0,
+    Close = 1,
+    Ping = 2,
+    Pong = 3,
+    Message = 4,
+    Upgrade = 5,
+    Noop = 6,
+}