fix: use runInShell=false for iperf3 binary execution and add logging

- Android toybox ash cannot exec ELF binaries even with -rwx permissions
- runInShell: true wraps in /system/bin/sh -c which refuses ELF
- runInShell: false calls execve() directly, bypassing sh
- Added debugPrint logging to capture actual output in logcat

lib/data/services/iperf3_service.dart

@@ -1,10 +1,11 @@
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:flutter/foundation.dart';
 import 'package:iperf3gui/domain/models/iperf3_result.dart';
 import 'package:iperf3gui/domain/models/test_configuration.dart';
 
-/// Service that runs iperf3 via [Process.spawn] and captures output.
+/// Service that runs iperf3 via [Process.start] and captures output.
 class Iperf3Service {
   final String _binaryPath;
 
@@ -17,31 +18,46 @@ class Iperf3Service {
   Future<Iperf3Result> run(TestConfiguration config) async {
     final args = config.toCliArgs();
 
+    debugPrint(
+      'Iperf3Service: running $_binaryPath ${args.join(" ")}',
+    );
+
     final process = await Process.start(
       _binaryPath,
       args,
-      runInShell: true,
+      runInShell: false, // must execve directly — Android sh can't exec ELF binaries
     );
 
     final stdoutBuffer = <int>[];
     final stderrBuffer = <int>[];
 
     final stdoutStream = process.stdout.transform(utf8.decoder).listen(
-      (data) => stdoutBuffer.addAll(utf8.encode(data)),
+      (data) {
+        stdoutBuffer.addAll(utf8.encode(data));
+        debugPrint('Iperf3Service stdout: $data');
+      },
     );
     final stderrStream = process.stderr.transform(utf8.decoder).listen(
-      (data) => stderrBuffer.addAll(utf8.encode(data)),
+      (data) {
+        stderrBuffer.addAll(utf8.encode(data));
+        debugPrint('Iperf3Service stderr: $data');
+      },
     );
 
-    await process.exitCode;
+    final exitCode = await process.exitCode;
+
     await stdoutStream.cancel();
     await stderrStream.cancel();
 
     final rawOutput = utf8.decode(stdoutBuffer) + utf8.decode(stderrBuffer);
 
+    debugPrint(
+      'Iperf3Service: finished with exit code $exitCode, output length ${rawOutput.length}',
+    );
+
     return Iperf3Result(
       rawOutput: rawOutput,
-      exitCode: await process.exitCode,
+      exitCode: exitCode,
     );
   }
 }