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_screen.dart

@@ -195,29 +195,31 @@ class _TestRunContentState extends State<_TestRunContent> {
             ),
             const SizedBox(height: 12),
 
-            // Live throughput figures, derived from the parsed interval series
+            // Live throughput figures. Every number here is read off the
+            // session; none are computed in the widget tree.
             Consumer<TestRunViewModel>(
               builder: (context, vm, _) {
-                final intervals = vm.intervals;
-                if (intervals.isEmpty) return const SizedBox.shrink();
-                final rates =
-                    intervals.map((i) => i.bitsPerSecond).toList(growable: false);
-                final latest = rates.last;
-                final peak = rates.reduce((a, b) => a > b ? a : b);
-                final mean = rates.reduce((a, b) => a + b) / rates.length;
-                final summary = vm.summary?.sent;
+                final session = vm.session;
+                final latest = session.latestBitsPerSecond;
+                final mean = session.meanBitsPerSecond;
+                final peak = session.peakBitsPerSecond;
+                if (latest == null || mean == null || peak == null) {
+                  return const SizedBox.shrink();
+                }
+                final total = session.summary?.sent;
                 return Padding(
                   padding: const EdgeInsets.only(bottom: 12),
                   child: Row(
                     mainAxisAlignment: MainAxisAlignment.spaceBetween,
                     children: [
                       _Stat(
-                        label: summary != null ? 'total' : 'now',
-                        value: formatBitrate(summary?.bitsPerSecond ?? latest),
+                        label: total != null ? 'total' : 'now',
+                        value: formatBitrate(total?.bitsPerSecond ?? latest),
                       ),
                       _Stat(label: 'avg', value: formatBitrate(mean)),
                       _Stat(label: 'peak', value: formatBitrate(peak)),
-                      _Stat(label: 'samples', value: '${rates.length}'),
+                      _Stat(
+                          label: 'samples', value: '${session.sampleCount}'),
                     ],
                   ),
                 );