fix: resolve argv[0] misparse and SIGSEGV hang in JNI bridge

Two root causes prevented iperf3 tests from running:

1. argv[0] bug: iperf_parse_arguments() expects argv[0] to be the
   program name. We passed Dart args directly, so argv[0] was '-c',
   which iperf3 misinterpreted as the program name → 'Argument parse
   error: must either be a client (-c) or server (-s)'. Fixed by
   prepending "iperf3" as argv[0] and shifting user args to argv[1+].

2. Signal handling: iperf_catch_sigend() installs handlers that use
   longjmp() to a jmp_buf, but we never initialized the buffer with
   setjmp(). Any signal during iperf_run_client() caused SIGSEGV or
   hang. Added setjmp/longjmp wrapper matching main.c's pattern,
   SIGPIPE=SIG_IGN, and a 30s connect_timeout to avoid DNS hangs.

Also extracted iperf_cleanup_and_signal() to deduplicate the
normal-return and longjmp paths. Added diagnostic logging to
IperfRunner.kt's library loading for faster future debugging.

android/app/src/main/kotlin/com/novoyuuparosk/iperf3gui/IperfRunner.kt

@@ -19,7 +19,14 @@ object IperfRunner : FlutterPlugin {
 
     init {
         // Load native library at class initialization
-        System.loadLibrary("iperf3jni")
+        try {
+            android.util.Log.i("IperfRunner", "Loading native library...")
+            System.loadLibrary("iperf3jni")
+            android.util.Log.i("IperfRunner", "Native library loaded successfully")
+        } catch (e: UnsatisfiedLinkError) {
+            android.util.Log.e("IperfRunner", "Failed to load native library: ${e.message}")
+            e.printStackTrace()
+        }
     }
 
     override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {