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/src/main/cpp/iperf/iperf_time.h

@@ -0,0 +1,49 @@
+/*
+ * iperf, Copyright (c) 2014-2018, The Regents of the University of
+ * California, through Lawrence Berkeley National Laboratory (subject
+ * to receipt of any required approvals from the U.S. Dept. of
+ * Energy).  All rights reserved.
+ *
+ * If you have questions about your rights to use or distribute this
+ * software, please contact Berkeley Lab's Technology Transfer
+ * Department at TTD@lbl.gov.
+ *
+ * NOTICE.  This software is owned by the U.S. Department of Energy.
+ * As such, the U.S. Government has been granted for itself and others
+ * acting on its behalf a paid-up, nonexclusive, irrevocable,
+ * worldwide license in the Software to reproduce, prepare derivative
+ * works, and perform publicly and display publicly.  Beginning five
+ * (5) years after the date permission to assert copyright is obtained
+ * from the U.S. Department of Energy, and subject to any subsequent
+ * five (5) year renewals, the U.S. Government is granted for itself
+ * and others acting on its behalf a paid-up, nonexclusive,
+ * irrevocable, worldwide license in the Software to reproduce,
+ * prepare derivative works, distribute copies to the public, perform
+ * publicly and display publicly, and to permit others to do so.
+ *
+ * This code is distributed under a BSD style license, see the LICENSE
+ * file for complete information.
+ */
+#ifndef __IPERF_TIME_H
+#define __IPERF_TIME_H
+
+#include <stdint.h>
+
+struct iperf_time {
+    uint32_t secs;
+    uint32_t usecs;
+};
+
+int iperf_time_now(struct iperf_time *time1);
+
+void iperf_time_add_usecs(struct iperf_time *time1, uint64_t usecs);
+
+int iperf_time_compare(struct iperf_time *time1, struct iperf_time *time2);
+
+int iperf_time_diff(struct iperf_time *time1, struct iperf_time *time2, struct iperf_time *diff);
+
+uint64_t iperf_time_in_usecs(struct iperf_time *time);
+
+double iperf_time_in_secs(struct iperf_time *time);
+
+#endif