perf(chart): redraw per display frame and cache the plot

[mrcsin]

Problem

A drag repainted the chart at about 30 frames per second and late (plot frame spacing p50 33.4 ms, pan step to
the next frame p50 24.4 ms, 2026-10-08, 8 pens), a hover over a still plot re-rendered it about 52 times per
second, and every hidden pen axis regenerated its ticks twice per frame. No thread was saturated; the costs that
scale with the pen count left little headroom for a 50-pen catalogue.

Change

• Frame-paced redraw. The view model raises RedrawRequested at once; the view coalesces requests into one
  TopLevel.RequestAnimationFrame, whose callback calls PlotControl.InvalidateVisual(). ChartRedrawSchedule
  and its 33 ms timer are deleted; RequestRedraw is UI-thread only, enforced by Avalonia.
• Cached plot layer. PlotControl sits alone inside the PlotLayer decorator, which carries a
  BitmapCache; the overlay repaints over the cached raster. The cache is on the wrapper because Avalonia 12 never
  refreshes a BitmapCache set on a self-drawing visual such as AvaPlot (charting.md#hover-and-the-plot-cache).
• Ticks for drawn axes only. LinearTickGenerator and LogTickGenerator generate nothing while hidden and keep
  their last two results (TickCache); ChartAxisBinder flips IsDrawn and IsVisible together under
  Plot.Sync, only on a change. With the active pen switched off the plot shows no horizontal gridlines.
• Debouncer. Admission runs on the data scheduler after the Sample/Throttle gates; Dispose completes the
  admission subject instead of disposing it.
• Data-area width is posted only when it changes.
• scripts/perf/trace-shares.py reports frame spacing, pan latency, compositor rate and tick cost, with
  --skip-seconds; testing-strategy.md#frame-cost states the stand checks, including the follow capture that
  catches a plot that stops redrawing.

Verification

• dotnet test SemiPlot.slnx: unit 1843, integration 147; dotnet format --verify-no-changes clean.
• Demo stand, Release, 8 pens, 2026-10-09: drag frame spacing p50 17.8 ms; follow mode without input one render
  per data tick; hover over a still plot about one render per second; theme switch repaints without a resize.

Per-task commits before collapse

8019cae perf(chart): cache the plot through a wrapper layer
b9c7f20 fix(chart): drop the bitmap cache that froze the plot
4f742d0 docs(plans): record the render headroom run
1035a14 refactor(chart): trim comments the docs already hold
d036710 refactor(chart): address smells review findings
c62d777 fix(chart): address review findings
25e2b7f docs(plans): close the render headroom tasks
cd20c20 docs(plans): record the render headroom checks
bc6db78 perf(chart): report the data-area width on change
c970c68 perf(chart): admit history requests past the gates
3b13199 perf(chart): publish no ticks from a hidden axis
b4a0443 perf(chart): generate ticks for drawn axes only
39e16c7 perf(chart): bitmap-cache the plot visual
ce36c10 perf(chart): pace the redraw by the display frame
9c4e925 chore(perf): measure frame cadence in trace-shares
4f601b6 docs(plans): add chart render headroom plan