Chart: keep a coarse layer in memory so a fast pan never shows an empty pane

Why

Scroll fast and the pane goes blank, then fills a moment later.

Why it goes blank

Each pen owns exactly one column buffer, and a history load replaces it whole: TrendPenState.LoadHistory builds a list and hands it over in one Line.ReplaceColumns(columns) (Chart/TrendPenState.cs:38-49, Chart/EnvelopeLine.cs:109-115). Rendering culls to the viewport (Chart/EnvelopePath.cs:13-27); pan past the buffer and no column is visible, so Render returns having drawn nothing (Chart/EnvelopeLine.cs:86-90). There is no second buffer and no fallback layer anywhere.

The wait is the debounce plus the read: a 150 ms trailing throttle merged with a 400 ms cap (Chart/TrendChartViewModel.cs:23-24, Chart/ChartHistoryRequestDebouncer.cs:46-65), and the read itself measured at about 0.5 s near the raw ceiling (docs/plans/backlog.md:28-35). Prefetch covers one window width on each side (Chart/HistoryPrefetch.cs:12), so a slow drag never notices and a fast one always does.

Wanted

The fix is already written down in docs/plans/backlog.md:28-35 and this issue adopts it:

• Keep the Minute layer and coarser in memory for the whole archive extent, read once at startup, one read per layer.
• Draw the exposed strip from the coarse buffer immediately, then swap the Raw columns in when they land.
• Widen the Raw margin to several windows, now that the frame no longer pays for buffer width (013f491).

What it costs

A coarse layer is 15 s per point (docs/architecture/data-integration.md:160-165). One year is about two million points per pen; fifty pens make that a hundred million, and at 32 bytes per column it does not fit in memory. So the startup read is bounded by something: a coarser layer for a long extent, or a column budget over the extent rather than a whole layer. #76 produces the numbers this decision needs; the plan should not guess them.

Acceptance

A headless test that pans faster than the debounce and asserts the plottable draws a non-empty path on every frame. Today no test would go red if the pane went blank forever.