Replication heatmap: click a cell to drill into the node-pair time series

[kylebernhardy]

The onboarding hint (AnalyticsOnboardingHint.tsx:47) already tells users "heatmap cells are clickable for drilldown", but cells have no click handler. Build the real feature: clicking a replication-latency cell opens a line chart of that source→target pair over the selected window. The pipeline already builds per-pair line series (replication-latency.tsx#buildLineSeries), so most of the data path exists — this is mainly wiring a click/Enter handler on the gridcells (the ARIA grid + roving tabindex is already in place) to a dialog reusing the expand-dialog pattern from ChartExpandButton.

If this isn't wanted, the hint copy needs rewording instead (tracked in the code-quality checklist issue).

Acceptance criteria

• Click (or Enter/Space on the focused gridcell — the roving-tabindex grid already exists) on a cell with data opens a dialog containing the node-pair latency line chart over the currently selected window and quantile.
• Suppressed/absent cells are not activatable and stay aria-disabled.
• The dialog reuses the ChartExpandButton dialog pattern (title source → target, export buttons optional).
• The onboarding-hint copy in AnalyticsOnboardingHint.tsx:47 ends up truthful either way.

Implementation notes

• pipeline/replication-latency.tsx#buildLineSeries already builds per-pair line series — the data path exists.
• Keep HeatmapMatrix generic: add an optional onCellSelect(row, col) prop; wire the dialog in the replication renderer, and extend the existing handleKeyDown for Enter/Space.

Filed from a Status tab code review.

Comment generated by kAIle (Claude Fable 5)