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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>