Allow users to choose to overlay a standard distribution over a numeric summary

This would draw std-dev tick lines and a low opacity std distribution curve. This should be an opt in feature, since it wouldn't necessarily make sense for all columns.

This is very low priority.

No files, tests, or entry points are named. Start by locating the numeric summary rendering and the configuration path for column-level options, then check how optional visual overlays are handled. Done means an opt-in standard-deviation distribution and tick lines appear for selected numeric columns without changing other summaries.

Written by the indexing model from the issue text.