feat(ch934x_serial): 添加串口写入功能的端口索引参数

- write方法新增可选的serialPortIndex参数，支持直接写入指定端口
- 传入端口索引时不会改变当前活跃端口，保持原有活跃端口行为
- 添加对负数端口索引的验证处理，直接返回零避免异常
- 更新文档说明多端口并发客户端应使用显式端口写入方式
- 增加相关单元测试覆盖新参数的各种使用场景
- 同步更新Android原生层实现以支持端口索引传递

lib/ch934x_serial.dart

@@ -29,16 +29,14 @@ class Ch934xSerial {
   // ---------------------------------------------------------------------------
 
   /// 获取所有已连接的 CH934X 设备信息。
-  Future<List<Ch934xDeviceInfo>> getDeviceList() =>
-      _platform.getDeviceList();
+  Future<List<Ch934xDeviceInfo>> getDeviceList() => _platform.getDeviceList();
 
   /// 获取指定设备序列号;非 CH934X 设备时返回 null。
   Future<String?> getSerialNumber(int deviceId) =>
       _platform.getSerialNumber(deviceId);
 
   /// 获取指定设备类型,取值见 [Ch934xDeviceType]。
-  Future<int> getDeviceType(int deviceId) =>
-      _platform.getDeviceType(deviceId);
+  Future<int> getDeviceType(int deviceId) => _platform.getDeviceType(deviceId);
 
   /// 获取指定设备的串口列表。
   Future<List<Ch934xSerialPortInfo>> getSerialPortList(
@@ -85,7 +83,11 @@ class Ch934xSerial {
   Future<Uint8List> read(int length) => _platform.read(length);
 
   /// 写入数据,返回实际写入的字节数。
-  Future<int> write(Uint8List data) => _platform.write(data);
+  ///
+  /// [serialPortIndex] 为空时沿用当前活跃串口；传入时直接写入指定端口，
+  /// 不会改变当前活跃串口。
+  Future<int> write(Uint8List data, {int? serialPortIndex}) =>
+      _platform.write(data, serialPortIndex: serialPortIndex);
 
   /// 构造一个持续从串口拉取数据的 `Stream<Uint8List>`。
   ///
@@ -180,12 +182,10 @@ class Ch934xSerial {
   Future<int> getCurrentMode() => _platform.getCurrentMode();
 
   /// 获取 GPIO 数量。
-  Future<int> getGpiocount(int deviceId) =>
-      _platform.getGpiocount(deviceId);
+  Future<int> getGpiocount(int deviceId) => _platform.getGpiocount(deviceId);
 
   /// 获取 GPIO 组数。
-  Future<int> getGpiogroup(int deviceId) =>
-      _platform.getGpiogroup(deviceId);
+  Future<int> getGpiogroup(int deviceId) => _platform.getGpiogroup(deviceId);
 
   /// 使能 GPIO。
   Future<bool> enableGpio({
@@ -222,12 +222,10 @@ class Ch934xSerial {
       );
 
   /// 查询设备是否已打开。
-  Future<bool> isConnected(int deviceId) =>
-      _platform.isConnected(deviceId);
+  Future<bool> isConnected(int deviceId) => _platform.isConnected(deviceId);
 
   /// 获取当前已打开的设备 ID 列表。
-  Future<List<int>> getConnectedDevices() =>
-      _platform.getConnectedDevices();
+  Future<List<int>> getConnectedDevices() => _platform.getConnectedDevices();
 
   /// 配置当前活跃串口的参数。
   Future<bool> setSerialParameter({

lib/ch934x_serial_method_channel.dart

@@ -14,8 +14,7 @@ import 'src/models/models.dart';
 class MethodChannelCh934xSerial extends Ch934xSerialPlatform {
   /// 测试时可被替换的 MethodChannel。
   @visibleForTesting
-  final MethodChannel methodChannel =
-      const MethodChannel('ch934x_serial');
+  final MethodChannel methodChannel = const MethodChannel('ch934x_serial');
 
   /// 通知 Dart 侧的异常事件流,用于支持 `setExceptionCallback`。
   final StreamController<Ch934xException> _exceptionController =
@@ -39,7 +38,8 @@ class MethodChannelCh934xSerial extends Ch934xSerialPlatform {
 
   @override
   Future<List<Ch934xDeviceInfo>> getDeviceList() async {
-    final raw = await methodChannel.invokeMethod<List<dynamic>>('getDeviceList');
+    final raw =
+        await methodChannel.invokeMethod<List<dynamic>>('getDeviceList');
     if (raw == null) {
       return const <Ch934xDeviceInfo>[];
     }
@@ -137,14 +137,15 @@ class MethodChannelCh934xSerial extends Ch934xSerialPlatform {
   }
 
   @override
-  Future<int> write(Uint8List data) async {
-    if (data.isEmpty) {
+  Future<int> write(Uint8List data, {int? serialPortIndex}) async {
+    if (data.isEmpty || (serialPortIndex != null && serialPortIndex < 0)) {
       return 0;
     }
-    final result = await methodChannel.invokeMethod<int>(
-      'write',
-      <String, Object>{'data': data},
-    );
+    final args = <String, Object>{'data': data};
+    if (serialPortIndex != null) {
+      args['serialPortIndex'] = serialPortIndex;
+    }
+    final result = await methodChannel.invokeMethod<int>('write', args);
     return result ?? 0;
   }
 
@@ -225,7 +226,8 @@ class MethodChannelCh934xSerial extends Ch934xSerialPlatform {
 
   @override
   Future<int> getCurrentMode() {
-    return methodChannel.invokeMethod<int>('getCurrentMode')
+    return methodChannel
+        .invokeMethod<int>('getCurrentMode')
         .then((v) => v ?? -1);
   }
 
@@ -301,7 +303,8 @@ class MethodChannelCh934xSerial extends Ch934xSerialPlatform {
 
   @override
   Future<List<int>> getConnectedDevices() async {
-    final raw = await methodChannel.invokeMethod<List<dynamic>>('getConnectedDevices');
+    final raw =
+        await methodChannel.invokeMethod<List<dynamic>>('getConnectedDevices');
     if (raw == null) return const <int>[];
     return raw.whereType<int>().toList(growable: false);
   }

lib/ch934x_serial_platform_interface.dart

@@ -79,7 +79,9 @@ abstract class Ch934xSerialPlatform extends PlatformInterface {
   Future<Uint8List> read(int length, {int? serialPortIndex});
 
   /// 向串口写入数据(对应 6.2.1 `UsbSerial.write`)。
-  Future<int> write(Uint8List data);
+  ///
+  /// [serialPortIndex] 为空时使用当前活跃端口；传入时直接写入指定端口。
+  Future<int> write(Uint8List data, {int? serialPortIndex});
 
   // ---------------------------------------------------------------------------
   // GPIO