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>

lib/ui/features/test_run/test_run_view_model.dart

@@ -39,6 +39,9 @@ class TestRunViewModel extends ChangeNotifier {
   /// Whether a test is currently in flight.
   bool get isRunning => _state == TestRunState.running;
 
+  /// Whether the most recent finished run ended because the user stopped it.
+  bool get wasStopped => _result?.stopped ?? false;
+
   /// Start a test with the given configuration.
   Future<void> runTest(TestConfiguration config) async {
     if (_state == TestRunState.running) return;