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>

test/domain/models/iperf3_result_test.dart

@@ -11,5 +11,15 @@ void main() {
       expect(result.rawOutput, equals('Test complete'));
       expect(result.exitCode, equals(0));
     });
+
+    test('stopped defaults to false', () {
+      const result = Iperf3Result(rawOutput: 'x', exitCode: 0);
+      expect(result.stopped, isFalse);
+    });
+
+    test('stopped is preserved when set', () {
+      const result = Iperf3Result(rawOutput: 'x', exitCode: 0, stopped: true);
+      expect(result.stopped, isTrue);
+    });
   });
 }