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/data/services/iperf3_service.dart

@@ -54,9 +54,13 @@ class Iperf3Service {
   /// Whether the native layer reported an error during this run.
   bool _sawError = false;
 
+  /// Whether [stop] was called for the current run.
+  bool _stopRequested = false;
+
   /// Cancel any running test.
   Future<void> stop() async {
     if (!_isRunning) return;
+    _stopRequested = true;
     try {
       await _channel.invokeMethod('stopIperf');
     } catch (e) {
@@ -83,6 +87,7 @@ class Iperf3Service {
 
     _isRunning = true;
     _sawError = false;
+    _stopRequested = false;
     _outputBuffer.clear();
 
     final args = config.toCliArgs();
@@ -157,7 +162,10 @@ class Iperf3Service {
           completer.complete(
             Iperf3Result(
               rawOutput: _transcript(),
-              exitCode: _sawError ? exitCodeNativeError : 0,
+              // A user-requested stop is a normal outcome, not a failure.
+              exitCode:
+                  (_sawError && !_stopRequested) ? exitCodeNativeError : 0,
+              stopped: _stopRequested,
             ),
           );
         }