fix(chart): let the render path rest when idle

[mrcsin]

Problem

Group F of docs/plans/20260929-window-per-process.md (Tasks 11-14):

• the chart's redraw ran through Sample(33 ms), a periodic timer that woke the UI thread every 33 ms for the chart's whole life, idle or not;
• RenderFinished read a view field on the render thread that the UI thread wrote;
• the column buffer shifted 100 000 columns on every append past its cap;
• a sidebar box and the chart each wrote a pen's visibility, kept apart by _isSettingVisibilityFromChart.

What changed

• ChartRedrawSchedule: one emission 33 ms ahead, later requests join it, nothing scheduled while no view subscribes, Dispose cancels a pending one.
• TrendChartView.ReportDataAreaWidthTo(TrendChartViewModel): each frame's width goes to the view model it was drawn for, on the UI thread; SetTargetColumnCount drops a repeat.
• EnvelopeLine trims MaxColumns / 10 at once.
• TrendPenState.IsVisible has a private setter and one writer, SetPenVisibility. The row box binds it OneWay and writes through ToggleVisibilityCommand; the group switch takes the same route. The row binds name, colour and unit through PenState.Pen, so a revised pen shows without a rebuild.
• Two tests that raced the code under load now wait for the event they assert on: ChartHistoryRequestCancellationTests (AwaitStart), and the settings restart tests in StartupFailureWindowTests, MainWindowViewTests, SettingsViewTests (the button's IsEffectivelyEnabled).
• CLAUDE.md, charting.md, trend-interaction.md, bench.md, testing-strategy.md, trend-feature-spec.md follow the code.

Verification

• dotnet build SemiPlot.slnx: 0 warnings. Unit 1591/1591, integration 138/138.
• Acceptance Evidence item 6 grep prints nothing; AppendingPastTheCapTrimsOneChunk and AnIdleChartSchedulesNoRedraw pass. The plan's "Verify it yourself" lists every new test.
• Manual run on the demo stand: sidebar boxes, group switch, pen unit and name edits, resize, idle CPU.

Per-task commits before collapse

33acadd docs(plans): record the render-path run
efbc7d8 refactor(chart): drop restating comments
90d1335 refactor(chart): address smells findings
762c4ff fix(chart): address review findings
1165db1 refactor(legend): give pen visibility one writer
6915c4d fix(chart): let an idle chart schedule no redraw
4eb4f6c fix(chart): keep the data-area width on the UI thread
f6c26f3 fix(chart): trim the column buffer in chunks

https://claude.ai/code/session_01PfZJa7neMMKPzycFR6iuhD