fix: ffi instance be singleton

ui/flutter/lib/api/native/ffi/libgopeed_ffi.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:ffi';
 
 import 'package:ffi/ffi.dart';
 
@@ -13,21 +14,25 @@ class LibgopeedFFi implements LibgopeedAbi {
   }
 
   @override
-  Future<String> create(String cfg) {
+  Future<void> init(String cfg) {
     final cfgPtr = cfg.toNativeUtf8();
     try {
-      final result = _libgopeed.Create(cfgPtr.cast());
-      return Future.value(result.cast<Utf8>().toDartString());
+      final result = _libgopeed.Init(cfgPtr.cast());
+      if (result != nullptr) {
+        throw Exception(
+            'Libgopeed init failed: ${result.cast<Utf8>().toDartString()}');
+      }
+      return Future.value();
     } finally {
       calloc.free(cfgPtr);
     }
   }
 
   @override
-  Future<String> invoke(int instance, String params) {
+  Future<String> invoke(String params) {
     final paramsPtr = params.toNativeUtf8();
     try {
-      final result = _libgopeed.Invoke(instance, paramsPtr.cast());
+      final result = _libgopeed.Invoke(paramsPtr.cast());
       return Future.value(result.cast<Utf8>().toDartString());
     } finally {
       calloc.free(paramsPtr);