feat: add Flutter desktop client for Rtty

desktop/test/e2e_ws_test.dart

@@ -0,0 +1,61 @@
+// 端到端集成测试：桌面端连接层 ↔ Rust 服务端。
+//
+// 运行前需先启动 Rust 服务端：cd .. && cargo run
+// 运行：flutter test test/e2e_ws_test.dart
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:xterm/xterm.dart';
+
+import '../lib/src/rtty_client.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  test('RttyClient connects to Rust server and renders command output',
+      () async {
+    final terminal = Terminal();
+    final client = RttyClient(terminal);
+
+    final states = <ConnState>[];
+    String? sessionId;
+    final raw = StringBuffer();
+    client.onStateChanged = (s) => states.add(s);
+    client.onReady = (id, cols, rows) => sessionId = '$id:$cols:$rows';
+    client.onRawData = (d) => raw.write(d);
+
+    await client.connect(host: '127.0.0.1', port: 8080);
+
+    // 等待连接就绪（socket 已连接 + 收到 ready 帧）。
+    final deadline = DateTime.now().add(const Duration(seconds: 5));
+    while ((!client.isConnected || sessionId == null) &&
+        DateTime.now().isBefore(deadline)) {
+      await Future<void>.delayed(const Duration(milliseconds: 100));
+    }
+
+    expect(client.isConnected, isTrue,
+        reason: '未能连接服务端 states=$states');
+    expect(sessionId, isNotNull,
+        reason: '未收到 ready 帧 states=$states');
+
+    // 发送命令，等待回显。
+    client.sendInput('echo RTTY_E2E_${DateTime.now().millisecondsSinceEpoch}\r\n');
+
+    String? matchedLine;
+    final outDeadline = DateTime.now().add(const Duration(seconds: 8));
+    while (DateTime.now().isBefore(outDeadline)) {
+      await Future<void>.delayed(const Duration(milliseconds: 150));
+      final text = raw.toString();
+      final lines = text.split('\n');
+      matchedLine = lines.where((l) => l.contains('RTTY_E2E')).lastOrNull;
+      if (matchedLine != null) break;
+    }
+
+    expect(matchedLine, isNotNull,
+        reason: '未捕获到命令回显。流末尾：'
+            '${raw.toString().split('').length > 200 ? raw.toString().substring(raw.toString().length - 200) : raw.toString()}');
+
+    await client.disconnect();
+    expect(client.state, ConnState.disconnected);
+  });
+}