feat(device): 实现下位机 JSON 协议（data model 对齐）

按 docs/下位机交互数据模型.md 重构串口协议层：

协议层
- 新增 DeviceMessage 模型，对应 message_id/type/ack/need_ack/data
- 新增 JsonProtocolService，4 字节大端长度前缀 + UTF-8 JSON 帧
- 删除原二进制协议（serial_protocol.dart）

服务层
- 新增 DeviceMessageService，集中收发并按 type 分发
- 重写 SerialRunner 为 JsonSerialRunner，使用 create_task/control 消息

数据模型
- DeviceState 增加 doorStatus/lightStatus/taskStatus/lastInfoAt
- 新增 DeviceInfoNotifier 订阅 device_info 上行
- 灯光按钮接通 light_control 消息

测试
- 新增 device_protocol_test.dart（14 用例）
- 修复 models_test.dart 残留的 Step mixSpeed/blowSpeed 错误

lib/features/device/services/device_message_service.dart

@@ -0,0 +1,147 @@
+import 'dart:async';
+import 'dart:typed_data';
+
+import 'serial_port_service.dart';
+import 'device_message.dart';
+import 'json_protocol.dart';
+
+/// 设备消息分发器
+///
+/// 集中处理与下位机之间的 JSON 消息收发：
+/// 1. 接收：监听串口原始字节，调用 [JsonProtocolService] 解码为 [DeviceMessage]，
+///    按 type 分发到对应订阅者；
+/// 2. 发送：把 [DeviceMessage] 编码为字节流写入串口。
+///
+/// 各业务方（SerialRunner / DeviceInfoNotifier / LightControl）只需关心
+/// 自己订阅/发送的消息类型，不必直接接触串口。
+class DeviceMessageService {
+  final SerialPortService _serial;
+  final JsonProtocolService _protocol;
+
+  StreamSubscription<Uint8List>? _rxSub;
+  StreamSubscription<String>? _errorSub;
+  StreamSubscription<SerialConnectionState>? _stateSub;
+
+  final Map<DeviceMessageType, List<void Function(DeviceMessage)>> _handlers = {};
+
+  /// 错误信息流（解码错误、串口异常等）
+  final _errorCtrl = StreamController<String>.broadcast();
+
+  /// 任何消息流的广播（用于调试 / 日志）
+  final _messageCtrl = StreamController<DeviceMessage>.broadcast();
+
+  DeviceMessageService({
+    required SerialPortService serial,
+    JsonProtocolService? protocol,
+  })  : _serial = serial,
+        _protocol = protocol ?? JsonProtocolService() {
+    _subscribe();
+  }
+
+  Stream<String> get onError => _errorCtrl.stream;
+  Stream<DeviceMessage> get onMessage => _messageCtrl.stream;
+
+  /// 串口连接状态变更（透传自 [SerialPortService]）
+  Stream<SerialConnectionState> get connectionStateChanges =>
+      _serial.connectionStateChanges;
+
+  /// 是否可发送消息（已连接且已初始化）
+  bool get canSend => _serial.isConnected;
+
+  /// 当前连接状态
+  SerialConnectionState get connectionState => _serial.state;
+
+  /// 生成下一个消息 ID
+  String nextId() => _protocol.nextId();
+
+  /// 订阅指定 [type] 的消息；返回取消订阅的函数
+  void Function() subscribe(DeviceMessageType type, void Function(DeviceMessage) handler) {
+    _handlers.putIfAbsent(type, () => <void Function(DeviceMessage)>[]);
+    _handlers[type]!.add(handler);
+    return () {
+      final list = _handlers[type];
+      if (list != null) list.remove(handler);
+    };
+  }
+
+  /// 发送消息到下位机；返回是否成功写入
+  Future<bool> send(DeviceMessage message) async {
+    if (!_serial.isConnected) {
+      _emitError('串口未连接，无法发送 ${message.type.wireName}');
+      return false;
+    }
+    try {
+      final bytes = _protocol.encode(message);
+      final written = await _serial.write(bytes);
+      if (written == 0) {
+        _emitError('写入失败：${message.type.wireName}');
+        return false;
+      }
+      _messageCtrl.add(message);
+      return true;
+    } catch (e) {
+      _emitError('编码异常: $e');
+      return false;
+    }
+  }
+
+  /// 释放资源
+  Future<void> dispose() async {
+    await _rxSub?.cancel();
+    await _errorSub?.cancel();
+    await _stateSub?.cancel();
+    _rxSub = _errorSub = _stateSub = null;
+    _handlers.clear();
+    await _errorCtrl.close();
+    await _messageCtrl.close();
+  }
+
+  // -- 私有方法 ---------------------------------------------------------
+
+  void _subscribe() {
+    _rxSub?.cancel();
+    _errorSub?.cancel();
+    _stateSub?.cancel();
+
+    _rxSub = _serial.onData.listen(_onData);
+    _errorSub = _serial.onError.listen(_emitError);
+    _stateSub = _serial.connectionStateChanges.listen((s) {
+      if (s == SerialConnectionState.disconnected) {
+        _protocol.reset();
+      }
+    });
+  }
+
+  void _onData(Uint8List data) {
+    if (data.isEmpty) return;
+    // 反复解析直到缓冲区无法形成完整帧
+    while (true) {
+      final (msg, consumed) = _protocol.tryDecode(data);
+      if (msg == null) {
+        if (consumed > 0 && data.length >= consumed) {
+          data = Uint8List.sublistView(data, consumed);
+        }
+        return;
+      }
+      data = Uint8List.sublistView(data, consumed);
+      _dispatch(msg);
+    }
+  }
+
+  void _dispatch(DeviceMessage msg) {
+    _messageCtrl.add(msg);
+    final list = _handlers[msg.type];
+    if (list == null) return;
+    for (final handler in List.of(list)) {
+      try {
+        handler(msg);
+      } catch (e) {
+        _emitError('消息处理异常 (${msg.type.wireName}): $e');
+      }
+    }
+  }
+
+  void _emitError(String message) {
+    if (!_errorCtrl.isClosed) _errorCtrl.add(message);
+  }
+}