refactor: depend on an Iperf3Runner interface instead of the service

The presentation layer imported Iperf3Service directly, so the UI was welded
to the MethodChannel transport: the view model named it, and the screen
constructed one. Dependency direction was already inward-only, but the UI
depended on an implementation rather than a contract.

Iperf3Runner states that contract in the domain layer in five members, in pure
Dart with no Flutter import, and Iperf3Service implements it. The view model
and screen now take an Iperf3Runner; main.dart is the only file that names a
concrete one, which makes it the composition root.

Two consequences worth having. The UI can be driven against an in-memory
runner with no MethodChannel, native library or device involved. And a test
double implements five stable members rather than tracking every public member
the transport happens to expose — the previous double, forced to implement the
concrete class, broke twice while this branch's work was going on.

Also drops flutter/foundation from the data layer: debugPrint gives way to
dart:developer, leaving flutter/services (where MethodChannel lives) as that
layer's only Flutter dependency.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/ui/features/test_run/test_run_view_model.dart

@@ -1,10 +1,10 @@
 import 'dart:async';
 
 import 'package:flutter/foundation.dart';
-import 'package:iperf3gui/data/services/iperf3_service.dart';
 import 'package:iperf3gui/domain/models/iperf3_event.dart';
 import 'package:iperf3gui/domain/models/iperf3_result.dart';
 import 'package:iperf3gui/domain/models/test_configuration.dart';
+import 'package:iperf3gui/domain/services/iperf3_runner.dart';
 import 'package:iperf3gui/ui/features/test_run/iperf3_event_formatter.dart';
 
 /// State of the test run.
@@ -12,10 +12,10 @@ enum TestRunState { idle, running, completed, error }
 
 /// ViewModel for managing test run state.
 ///
-/// Subscribes to [Iperf3Service.events] so the transcript and the interval
+/// Subscribes to [Iperf3Runner.events] so the transcript and the interval
 /// series both grow while the test runs, rather than appearing at the end.
 class TestRunViewModel extends ChangeNotifier {
-  final Iperf3Service iperf3Service;
+  final Iperf3Runner iperf3Service;
 
   /// How often to notify listeners while output is arriving. iperf3 can report
   /// several times a second across many streams, and each rebuild re-renders