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/domain/models/iperf3_result.dart

@@ -3,8 +3,13 @@ class Iperf3Result {
   final String rawOutput;
   final int exitCode;
 
+  /// Whether the run ended because the user asked it to stop, rather than
+  /// finishing its configured duration or failing.
+  final bool stopped;
+
   const Iperf3Result({
     required this.rawOutput,
     required this.exitCode,
+    this.stopped = false,
   });
 }