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_screen.dart

@@ -166,7 +166,10 @@ class _TestRunContentState extends State<_TestRunContent> {
                 final (String label, Color color) = switch (vm.state) {
                   TestRunState.idle => ('Ready', scheme.onSurfaceVariant),
                   TestRunState.running => ('Running…', scheme.primary),
-                  TestRunState.completed => ('Test complete', scheme.primary),
+                  TestRunState.completed => (
+                      vm.wasStopped ? 'Stopped' : 'Test complete',
+                      vm.wasStopped ? scheme.onSurfaceVariant : scheme.primary,
+                    ),
                   TestRunState.error => (
                       'Error${vm.result != null ? ' (exit ${vm.result!.exitCode})' : ''}',
                       scheme.error,