refactor: move run accumulation out of the view model into the domain

The view model was doing two jobs. Alongside presentation state it held what
happened during a run — the interval series, the rule that iperf3's omitted
warm-up intervals do not count, the retained end summary — and the screen
computed mean and peak inline in a widget builder. All of that is the part
that has to be correct, and all of it would have been rewritten by anyone
replacing the UI.

Iperf3Session accumulates a run in the domain layer: samples, totals, errors,
whether it was stopped, and the statistics over them. The view model now feeds
it events and keeps only a formatted transcript and the repaint schedule; the
screen reads every displayed number off the session and derives none.

Two details the session now owns rather than leaving to a caller. Omitted
intervals are excluded, so a warm-up cannot drag the mean down. And
totalRetransmits stays null when no interval reported a count — UDP does not
measure retransmits, and "not measured" must not render as zero.

The aggregation tests move with the logic, into a pure-Dart test with no
widgets or bindings involved.

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

lib/ui/features/test_run/test_run_view_model.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:flutter/foundation.dart';
 import 'package:iperf3gui/domain/models/iperf3_event.dart';
 import 'package:iperf3gui/domain/models/iperf3_result.dart';
+import 'package:iperf3gui/domain/models/iperf3_session.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';
@@ -12,8 +13,10 @@ enum TestRunState { idle, running, completed, error }
 
 /// ViewModel for managing test run state.
 ///
-/// Subscribes to [Iperf3Runner.events] so the transcript and the interval
-/// series both grow while the test runs, rather than appearing at the end.
+/// Presentation only. What happened during a run is accumulated by
+/// [Iperf3Session] in the domain layer; this class subscribes to
+/// [Iperf3Runner.events], feeds that session, keeps a formatted transcript for
+/// display, and decides when to repaint.
 class TestRunViewModel extends ChangeNotifier {
   final Iperf3Runner iperf3Service;
 
@@ -41,24 +44,14 @@ class TestRunViewModel extends ChangeNotifier {
 
   final StringBuffer _transcript = StringBuffer();
 
-  /// Human-readable output accumulated so far.
+  /// Human-readable output accumulated so far. Display only — the structured
+  /// record of the run is [session].
   String get output => _transcript.toString();
 
-  final List<Iperf3Interval> _intervals = [];
+  final Iperf3Session _session = Iperf3Session();
 
-  /// Per-interval throughput samples, in arrival order. Structured, so a chart
-  /// can be driven from this without re-parsing text.
-  List<Iperf3Interval> get intervals => List.unmodifiable(_intervals);
-
-  Iperf3Start? _start;
-
-  /// Connection details reported at the start of the run.
-  Iperf3Start? get start => _start;
-
-  Iperf3End? _summary;
-
-  /// Totals reported at the end of the run.
-  Iperf3End? get summary => _summary;
+  /// The accumulated run: interval series, totals, errors and their statistics.
+  Iperf3Session get session => _session;
 
   /// Whether a test is currently in flight.
   bool get isRunning => _state == TestRunState.running;
@@ -118,26 +111,12 @@ class TestRunViewModel extends ChangeNotifier {
   void _resetRunData() {
     _result = null;
     _transcript.clear();
-    _intervals.clear();
-    _start = null;
-    _summary = null;
+    _session.clear();
     _dirty = false;
   }
 
   void _onEvent(Iperf3Event event) {
-    switch (event) {
-      case Iperf3Start e:
-        _start = e;
-      case Iperf3Interval e:
-        // Omitted intervals are warm-up (-O) and are not part of the series.
-        if (!e.omitted) _intervals.add(e);
-      case Iperf3End e:
-        _summary = e;
-      case Iperf3Error _:
-      case Iperf3Stopped _:
-      case Iperf3LogLine _:
-        break;
-    }
+    _session.add(event);
 
     final line = formatIperf3Event(event);
     if (line != null && line.isNotEmpty) {