Stress-window queries

Headline fields report the worst observed condition in a seven-day window. A stress-window query lets you choose the period instead — the worst hour, the worst day of a specific rotation, the worst point since recording began.

This is the query that matters for setting protocol parameters. A liquidation bonus tuned to the average book is tuned to conditions under which the bonus was never going to matter.

The answer is bounded by the record. Asking for a window longer than `days_observed` returns an error rather than a padded number.