fix: report a user-requested stop as stopped rather than failed

Stopping a test shuts down the control socket, so iperf_run_client() returns
-1 with "unable to send control message ... Broken pipe". That is the
expected consequence of the stop, not a fault, but the bridge read the
negative return as a failure — so pressing STOP left the UI showing
"Error (exit 1)" above a broken-pipe message.

The bridge now consults its own g_stop flag before classifying the outcome,
and the distinction travels up as Iperf3Result.stopped so the status line can
say "Stopped". A stop is reported with exit code 0: the run ended the way the
user asked it to.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

android/app/src/main/cpp/iperf_jni.c

@@ -251,8 +251,14 @@ static void iperf_cleanup_and_signal(JNIEnv *env,
     retire_test();
     free_argv(argv, argc);
 
+    /* A user-requested stop shuts down the control socket, so
+     * iperf_run_client() returns -1 with "Broken pipe" — expected, not a
+     * failure. Check g_stop so that path is reported as a stop, not an error. */
+    int user_stopped = stopped || g_stop;
+    g_stop = 0;
+
     /* Signal result to Java */
-    if (stopped) {
+    if (user_stopped) {
         signal_output(env, state, "[iPerf JNI] Test was stopped by user.\n");
     } else if (ret < 0) {
         char errbuf[256];