docs(webrtc_controller_flutter): 更新项目文档以反映重构后的架构

AGENTS.md 与 README.md 同步更新：根据实际代码结构重写目录树、技术栈、架构分层及编码规范，移除旧版内联示例并补充新的开发约定与代码生成命令。

webrtc_controller_flutter/lib/features/connection/domain/signal_message.dart

@@ -0,0 +1,58 @@
+import 'dart:convert';
+
+import 'package:freezed_annotation/freezed_annotation.dart';
+
+part 'signal_message.freezed.dart';
+part 'signal_message.g.dart';
+
+/// 信令消息模型，对应 Android 端的 SignalMessage。
+///
+/// 字段含义：
+/// - [type]        消息类型：REGISTER / OFFER / ANSWER / ICE_CANDIDATE
+/// - [fromDeviceId] 发送方设备 ID
+/// - [toDeviceId]   接收方设备 ID
+/// - [deviceType]   设备类型：CONTROLLER / CONTROLLED
+/// - [payload]      JSON 字符串形式的负载（SDP / ICE 候选等）
+/// - [authType]     鉴权类型：CODE（动态验证码）/ PASSWORD（固定密码）
+/// - [authValue]    鉴权值：动态验证码或固定密码
+@freezed
+class SignalMessage with _$SignalMessage {
+  const SignalMessage._();
+
+  const factory SignalMessage({
+    String? type,
+    String? fromDeviceId,
+    String? toDeviceId,
+    String? deviceType,
+    String? payload,
+    String? authType,
+    String? authValue,
+  }) = _SignalMessage;
+
+  factory SignalMessage.fromJson(Map<String, dynamic> json) =>
+      _$SignalMessageFromJson(json);
+
+  /// 便捷构造方法：payload 为任意 Map，会自动序列化为 JSON 字符串。
+  factory SignalMessage.withPayload({
+    required String type,
+    required String fromDeviceId,
+    required String toDeviceId,
+    required String deviceType,
+    required Map<String, dynamic> payload,
+    String? authType,
+    String? authValue,
+  }) {
+    return SignalMessage(
+      type: type,
+      fromDeviceId: fromDeviceId,
+      toDeviceId: toDeviceId,
+      deviceType: deviceType,
+      payload: jsonEncode(payload),
+      authType: authType,
+      authValue: authValue,
+    );
+  }
+
+  @override
+  String toString() => jsonEncode(toJson());
+}