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)

android/app/build.gradle.kts

@@ -22,11 +22,14 @@ android {
         minSdk = flutter.minSdkVersion
         targetSdk = flutter.targetSdkVersion
         // Uses the version code from pubspec.yaml. When using split APKs, 1000 * ABI_VERSION
-        // is added automatically by Flutter. (https://developer.android.com/studio/build/configure-apk-splits#configure-APK-versions)
+        // is added automatically by Flutter. (https://developer.android.com/studio/build/configure-apk-splits#configure-apk-versions)
         // You can force using the value of versionCode by specifying the `-P force-version-code-ignoring-abi=true`
         // flag during build.
         versionCode = flutter.versionCode
         versionName = flutter.versionName
+        ndk {
+            abiFilters += listOf("armeabi-v7a", "arm64-v8a")
+        }
     }
 
     buildTypes {
@@ -36,6 +39,13 @@ android {
             signingConfig = signingConfigs.getByName("debug")
         }
     }
+
+    externalNativeBuild {
+        cmake {
+            path = file("src/main/cpp/CMakeLists.txt")
+            version = "3.22.1"
+        }
+    }
 }
 
 kotlin {