feat: switch iperf3 to JNI with 3.19 source

- Switch from bundled iperf3 binary (Process.start) to JNI compilation
- iperf3 3.21 source API was incompatible (start_time, iperf_strerror, etc.)
- iperf3 3.19 source is proven to work (used by CellularLab)
- CMakeLists.txt updated: config.h generated into cpp/iperf/ for correct include paths
- iperf_jni.c fixed for 3.19 API (no start_time on iperf_test)
- getTestProgress() returns g_test->duration as fallback (no real-time elapsed tracking yet)
- NDK CMake pinned to 3.22.1
- UI layer updated: removed Iperf3BinaryPathProvider references
- Tests updated: TestRunViewModelTest matches new service interface
- pubspec.yaml: removed iperf3 assets bundle, added http dependency
- AndroidManifest: INTERNET permission (needed for hostname resolution)

lib/main.dart

@@ -1,10 +1,13 @@
 import 'package:flutter/material.dart';
-import 'package:iperf3gui/data/services/iperf3_binary_path_provider.dart';
+import 'package:iperf3gui/data/services/iperf3_service.dart';
 import 'package:iperf3gui/ui/features/test_run/test_run_screen.dart';
 
+/// Application entry point.
+///
+/// iperf3 is now compiled as a native shared library (libiperf3jni.so)
+/// and accessed via JNI — no binary extraction or chmod needed.
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
-  await Iperf3BinaryPathProvider.initialize();
   runApp(const Iperf3GuiApp());
 }