Enhance USB serial plugin for Android and iPadOS with DriverKit support

- Updated README.md to reflect platform support for iPadOS.
- Implemented isDriverAvailable method in FlutterUsbSerialPlugin for DriverKit readiness check.
- Added DriverKitSerialClient C source and header files for iPadOS integration.
- Created Swift Package for iPadOS support.
- Updated pubspec.yaml and flutter_usb_serial.dart to clarify platform compatibility.
- Enhanced tests to verify isDriverAvailable functionality.

Signed-off-by: Tony <tonylu@tony-cloud.com>

lib/flutter_usb_serial.dart

@@ -1,7 +1,8 @@
-/// Flutter plugin for USB serial communication on Android.
+/// Flutter plugin for USB serial communication on Android and iPadOS.
 ///
 /// Uses `usb-serial-for-android` (https://github.com/mik3y/usb-serial-for-android)
-/// as the underlying Android library.
+/// as the underlying Android library. iPadOS uses an app-embedded DriverKit
+/// extension supplied by the host application.
 library;
 
 import 'dart:async';

lib/src/flutter_usb_serial.dart

@@ -2,7 +2,8 @@ part of '../flutter_usb_serial.dart';
 
 /// Main entry point for the flutter_usb_serial plugin.
 ///
-/// Only supported on Android.
+/// Supported on Android and on iPadOS through an app-embedded DriverKit
+/// extension.
 class FlutterUsbSerial {
   FlutterUsbSerial._();
 
@@ -18,6 +19,15 @@ class FlutterUsbSerial {
     return _rawEventStream!;
   }
 
+  /// Whether the native USB serial backend is ready.
+  ///
+  /// Android always provides its USB host backend. On iPadOS this probes for
+  /// the DriverKit user-client service, so it is `false` until the matching
+  /// app-embedded driver has loaded successfully.
+  static Future<bool> isDriverAvailable() async {
+    return await _methods.invokeMethod<bool>('isDriverAvailable') ?? false;
+  }
+
   /// Returns the list of currently attached USB serial devices.
   static Future<List<UsbSerialDevice>> listDevices() async {
     final result = await _methods.invokeMethod<List<Object?>>('listDevices');
@@ -70,21 +80,24 @@ class FlutterUsbSerial {
 
     final dataStream = _eventStream
         .where((e) => e['portId'] == portId)
-        .transform(StreamTransformer<Map<Object?, Object?>, Uint8List>.fromHandlers(
-          handleData: (e, sink) {
-            if (e['type'] == 'data') {
-              final raw = e['data'];
-              if (raw is Uint8List) {
-                sink.add(raw);
-              } else if (raw is List) {
-                sink.add(Uint8List.fromList(raw.cast<int>()));
+        .transform(
+          StreamTransformer<Map<Object?, Object?>, Uint8List>.fromHandlers(
+            handleData: (e, sink) {
+              if (e['type'] == 'data') {
+                final raw = e['data'];
+                if (raw is Uint8List) {
+                  sink.add(raw);
+                } else if (raw is List) {
+                  sink.add(Uint8List.fromList(raw.cast<int>()));
+                }
+              } else if (e['type'] == 'error') {
+                sink.addError(
+                  Exception(e['message'] as String? ?? 'USB IO error'),
+                );
               }
-            } else if (e['type'] == 'error') {
-              sink.addError(Exception(
-                  e['message'] as String? ?? 'USB IO error'));
-            }
-          },
-        ));
+            },
+          ),
+        );
 
     return UsbSerialPort._(portId, dataStream);
   }