feat: SSL/TLS support for both client and server

- ProtobufServer: named constructor .secure() for SSL mode
  - SecurityContext parameter for certificate/key loading
  - SecureServerSocket 분기 처리, stop()에서 양쪽 close
  - isSecure getter 추가
- ProtobufClient: static factory 메서드 추가
  - connect() — plain TCP
  - connectSecure() — SSL/TLS (SecureSocket extends Socket이라 내부 무수정)
- 테스트: SSL 그룹 4개 추가 (총 13개, 전부 통과)
  - 자체 서명 인증서로 실제 SSL 연결 검증
  - plain / SSL 동일 시나리오 병렬 커버

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

dart/lib/src/protobuf_client.dart

@@ -14,6 +14,32 @@ abstract class ProtobufClient extends Communicator {
   late int _heartbeatIntervalTime;
   late int _heartbeatWaitTime;
   final Socket _socket;
+
+  /// Plain TCP connection.
+  ///
+  /// ```dart
+  /// final socket = await ProtobufClient.connect('localhost', 9090);
+  /// final client = MyClient(socket);
+  /// ```
+  static Future<Socket> connect(String host, int port) =>
+      Socket.connect(host, port);
+
+  /// SSL/TLS connection.
+  ///
+  /// ```dart
+  /// final socket = await ProtobufClient.connectSecure('example.com', 9090);
+  /// final client = MyClient(socket);
+  /// ```
+  ///
+  /// For self-signed or custom certificates, provide a [SecurityContext]:
+  /// ```dart
+  /// final ctx = SecurityContext()..setTrustedCertificates('ca.crt');
+  /// final socket = await ProtobufClient.connectSecure('host', 9090, context: ctx);
+  /// ```
+  static Future<Socket> connectSecure(String host, int port,
+          {SecurityContext? context}) =>
+      SecureSocket.connect(host, port,
+          context: context ?? SecurityContext.defaultContext);
   late ResponseChecker? _heartbeatChecker = null;
   int? _length = null;
   bool _isAlive = false;

dart/lib/src/protobuf_server.dart

@@ -8,28 +8,54 @@ import 'protobuf_client.dart';
 abstract class ProtobufServer {
   List<ProtobufClient> _clientList = [];
   bool _started = false;
-  late ServerSocket? _server;
+  ServerSocket? _server;
+  SecureServerSocket? _secureServer;
   final String _host;
   final int _port;
+  final SecurityContext? _securityContext;
   final ProtobufClient Function(Socket) _createNewClient;
 
   bool get started => _started;
+  bool get isSecure => _securityContext != null;
 
-  ProtobufServer(this._host, this._port, this._createNewClient);
+  /// Plain TCP server.
+  ProtobufServer(this._host, this._port, this._createNewClient)
+      : _securityContext = null;
+
+  /// SSL/TLS server.
+  ///
+  /// [securityContext] must have a certificate and private key loaded:
+  /// ```dart
+  /// final ctx = SecurityContext()
+  ///   ..useCertificateChain('server.crt')
+  ///   ..usePrivateKey('server.key');
+  /// final server = MyServer.secure('0.0.0.0', 9090, ctx, (s) => MyClient(s));
+  /// ```
+  ProtobufServer.secure(
+      this._host, this._port, SecurityContext securityContext, this._createNewClient)
+      : _securityContext = securityContext;
 
   Future start() async {
-    _server = await ServerSocket.bind(_host, _port);
-    _started = true;
-    _server?.listen((Socket socket) {
-      var client = _createNewClient(socket);
-      client.addDisconnectListener(onDisconnectedClient);
-      client.addListener<HeartBeat>((_) => onClientHeartBeat(client));
-      _clientList.add(client);
-      onClientConnected(client);
-    });
+    if (_securityContext != null) {
+      _secureServer = await SecureServerSocket.bind(_host, _port, _securityContext!);
+      _started = true;
+      _secureServer?.listen(_onClientSocket);
+    } else {
+      _server = await ServerSocket.bind(_host, _port);
+      _started = true;
+      _server?.listen(_onClientSocket);
+    }
     return Future.value();
   }
 
+  void _onClientSocket(Socket socket) {
+    var client = _createNewClient(socket);
+    client.addDisconnectListener(onDisconnectedClient);
+    client.addListener<HeartBeat>((_) => onClientHeartBeat(client));
+    _clientList.add(client);
+    onClientConnected(client);
+  }
+
   // implement in child
   void onClientConnected(ProtobufClient client);
 
@@ -50,6 +76,7 @@ abstract class ProtobufServer {
 
   Future stop() async {
     await _server?.close();
+    await _secureServer?.close();
     _started = false;
     return Future.value();
   }

dart/test/certs/server.crt

@@ -0,0 +1,19 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----

dart/test/certs/server.key

@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----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+-----END PRIVATE KEY-----

dart/test/socket_test.dart

@@ -5,7 +5,10 @@ import 'package:test/test.dart';
 import 'package:toki_socket/toki_socket.dart';
 
 const _testPort = 19090;
+const _testPortSsl = 19091;
 const _host = 'localhost';
+const _certPath = 'test/certs/server.crt';
+const _keyPath = 'test/certs/server.key';
 
 // ── 테스트 전용 픽스처 ──────────────────────────────────────────
 
@@ -29,10 +32,26 @@ class _TestServer extends ProtobufServer {
   }
 }
 
+class _TestServerSsl extends ProtobufServer {
+  final receivedMessages = <TestData>[];
+  final connectedClients = <ProtobufClient>[];
+
+  _TestServerSsl(SecurityContext ctx)
+      : super.secure(_host, _testPortSsl, ctx, (socket) => _TestClient(socket));
+
+  @override
+  void onClientConnected(ProtobufClient client) {
+    connectedClients.add(client);
+    client.addListener<TestData>((data) => receivedMessages.add(data));
+  }
+}
+
 // ── 테스트 ──────────────────────────────────────────────────────
 
 void main() {
-  group('ProtobufServer', () {
+  // ── Plain TCP ────────────────────────────────────────────────
+
+  group('ProtobufServer (plain)', () {
     late _TestServer server;
 
     setUp(() async {
@@ -46,19 +65,19 @@ void main() {
 
     test('서버가 정상 시작된다', () {
       expect(server.started, isTrue);
+      expect(server.isSecure, isFalse);
     });
   });
 
-  group('ProtobufClient', () {
+  group('ProtobufClient (plain)', () {
     late _TestServer server;
-    late Socket clientSocket;
     late _TestClient client;
 
     setUp(() async {
       server = _TestServer();
       await server.start();
-      clientSocket = await Socket.connect(_host, _testPort);
-      client = _TestClient(clientSocket);
+      final socket = await ProtobufClient.connect(_host, _testPort);
+      client = _TestClient(socket);
       await Future.delayed(const Duration(milliseconds: 100));
     });
 
@@ -73,11 +92,9 @@ void main() {
     });
 
     test('TestData 메시지를 서버가 수신한다', () async {
-      final msg = TestData()
+      await client.send(TestData()
         ..index = 42
-        ..message = 'hello toki-socket';
-
-      await client.send(msg);
+        ..message = 'hello toki-socket');
       await Future.delayed(const Duration(milliseconds: 200));
 
       expect(server.receivedMessages, hasLength(1));
@@ -104,8 +121,7 @@ void main() {
       client.addListener<TestData>((data) => completer.complete(data));
 
       await Future.delayed(const Duration(milliseconds: 100));
-      final serverClient = server.connectedClients.first;
-      await serverClient.send(TestData()
+      await server.connectedClients.first.send(TestData()
         ..index = 99
         ..message = 'from server');
 
@@ -115,17 +131,10 @@ void main() {
     });
 
     test('nonce가 송신마다 증가한다', () async {
-      final receivedNonces = <int>[];
-      server.connectedClients.first.addListener<TestData>((data) {});
-
-      // 서버에서 수신한 PacketBase의 nonce를 검증하기 위해
-      // 클라이언트에서 3개의 메시지를 순서대로 전송
       for (var i = 1; i <= 3; i++) {
         await client.send(TestData()..index = i);
       }
       await Future.delayed(const Duration(milliseconds: 200));
-
-      // 3개 수신 확인 (nonce 단조 증가는 wire level 검증 — 여기선 수신 순서로 간접 확인)
       expect(server.receivedMessages, hasLength(3));
     });
 
@@ -137,18 +146,107 @@ void main() {
 
       client.dispose();
       await completer.future.timeout(const Duration(seconds: 2));
-      // completer가 완료되면 disconnect 콜백이 정상 호출된 것
       expect(completer.isCompleted, isTrue);
     });
 
     test('HeartBeat interval 동안 연결이 유지된다', () async {
-      // heartbeat interval(5초)보다 짧은 시간 대기 후 메시지 송수신 확인
       await Future.delayed(const Duration(seconds: 2));
-
-      await client.send(TestData()..index = 1..message = 'alive check');
+      await client.send(TestData()
+        ..index = 1
+        ..message = 'alive check');
       await Future.delayed(const Duration(milliseconds: 200));
-
       expect(server.receivedMessages, hasLength(1));
     });
   });
+
+  // ── SSL/TLS ──────────────────────────────────────────────────
+
+  group('ProtobufServer (SSL)', () {
+    late _TestServerSsl server;
+
+    setUp(() async {
+      final ctx = SecurityContext()
+        ..useCertificateChain(_certPath)
+        ..usePrivateKey(_keyPath);
+      server = _TestServerSsl(ctx);
+      await server.start();
+    });
+
+    tearDown(() async {
+      await server.stop();
+    });
+
+    test('SSL 서버가 정상 시작된다', () {
+      expect(server.started, isTrue);
+      expect(server.isSecure, isTrue);
+    });
+  });
+
+  group('ProtobufClient (SSL)', () {
+    late _TestServerSsl server;
+    late _TestClient client;
+
+    setUp(() async {
+      final serverCtx = SecurityContext()
+        ..useCertificateChain(_certPath)
+        ..usePrivateKey(_keyPath);
+      server = _TestServerSsl(serverCtx);
+      await server.start();
+
+      final clientCtx = SecurityContext()
+        ..setTrustedCertificates(_certPath);
+      final socket = await ProtobufClient.connectSecure(
+        _host, _testPortSsl,
+        context: clientCtx,
+      );
+      client = _TestClient(socket);
+      await Future.delayed(const Duration(milliseconds: 100));
+    });
+
+    tearDown(() async {
+      client.dispose();
+      await Future.delayed(const Duration(milliseconds: 100));
+      await server.stop();
+    });
+
+    test('SSL 클라이언트가 서버에 연결된다', () {
+      expect(server.connectedClients, isNotEmpty);
+    });
+
+    test('SSL TestData 메시지를 서버가 수신한다', () async {
+      await client.send(TestData()
+        ..index = 7
+        ..message = 'hello over ssl');
+      await Future.delayed(const Duration(milliseconds: 200));
+
+      expect(server.receivedMessages, hasLength(1));
+      expect(server.receivedMessages.first.index, equals(7));
+      expect(server.receivedMessages.first.message, equals('hello over ssl'));
+    });
+
+    test('SSL 서버에서 클라이언트로 메시지를 전송한다', () async {
+      final completer = Completer<TestData>();
+      client.addListener<TestData>((data) => completer.complete(data));
+
+      await Future.delayed(const Duration(milliseconds: 100));
+      await server.connectedClients.first.send(TestData()
+        ..index = 55
+        ..message = 'from ssl server');
+
+      final received = await completer.future.timeout(const Duration(seconds: 2));
+      expect(received.index, equals(55));
+      expect(received.message, equals('from ssl server'));
+    });
+
+    test('SSL disconnect 시 서버 콜백이 호출된다', () async {
+      final completer = Completer<void>();
+      server.connectedClients.first.addDisconnectListener((_) {
+        if (!completer.isCompleted) completer.complete();
+      });
+
+      client.dispose();
+      await completer.future.timeout(const Duration(seconds: 2));
+      expect(completer.isCompleted, isTrue);
+    });
+  });
 }