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_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);
   }