feat: add domain models for test configuration and results

lib/domain/models/iperf3_result.dart

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+/// Raw result from an iperf3 execution.
+class Iperf3Result {
+  final String rawOutput;
+  final int exitCode;
+
+  const Iperf3Result({
+    required this.rawOutput,
+    required this.exitCode,
+  });
+}

lib/domain/models/test_configuration.dart

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+/// Configuration for an iperf3 client test.
+class TestConfiguration {
+  final String server;
+  final int port;
+  final Duration duration;
+  final int interval;
+  final int parallelStreams;
+
+  const TestConfiguration({
+    this.server = 'iperf.es.net',
+    this.port = 5201,
+    this.duration = const Duration(seconds: 10),
+    this.interval = 1,
+    this.parallelStreams = 1,
+  });
+
+  /// Convert to iperf3 CLI arguments.
+  List<String> toCliArgs() {
+    return <String>[
+      '-c', server,
+      '-p', '$port',
+      '-t', '${duration.inSeconds}',
+      '-i', '$interval',
+      '-P', '$parallelStreams',
+    ];
+  }
+
+  /// Validate the configuration.
+  bool get isValid {
+    if (server.isEmpty) return false;
+    if (port < 1 || port > 65535) return false;
+    if (duration.inSeconds < 1) return false;
+    if (interval < 1) return false;
+    if (parallelStreams < 1) return false;
+    return true;
+  }
+}

test/domain/models/iperf3_result_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:iperf3gui/domain/models/iperf3_result.dart';
+
+void main() {
+  group('Iperf3Result', () {
+    test('can be instantiated with raw output', () {
+      const result = Iperf3Result(
+        rawOutput: 'Test complete',
+        exitCode: 0,
+      );
+      expect(result.rawOutput, equals('Test complete'));
+      expect(result.exitCode, equals(0));
+    });
+  });
+}

test/domain/models/test_configuration_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:iperf3gui/domain/models/test_configuration.dart';
+
+void main() {
+  group('TestConfiguration', () {
+    test('defaults produce a valid configuration', () {
+      const config = TestConfiguration();
+      expect(config.isValid, isTrue);
+    });
+
+    test('empty server is invalid', () {
+      const config = TestConfiguration(server: '');
+      expect(config.isValid, isFalse);
+    });
+
+    test('port out of range is invalid', () {
+      expect(const TestConfiguration(port: 0).isValid, isFalse);
+      expect(const TestConfiguration(port: 65536).isValid, isFalse);
+    });
+
+    test('CLI args contain server and port', () {
+      const config = TestConfiguration(server: '192.168.1.1', port: 1234);
+      final args = config.toCliArgs();
+      expect(args, contains('-c'));
+      expect(args, contains('192.168.1.1'));
+      expect(args, contains('-p'));
+      expect(args, contains('1234'));
+    });
+  });
+}