Historical timezone overlay coverage report

Machine-generated report of what the historical-timezone correction overlay covers and what it does not. Regenerated from the same overlay.json used at request time, so it cannot drift from the live dataset.

covered lists every overlay row (region, local date range, zones, offsets, and its precision sourcing grade: 'statutory', 'documented', or 'inferred'). gaps lists, per region the overlay claims any coverage for, the uncovered date spans inside 1883-11-18 to 1970-01-01. Everything outside the polygons in covered, for every date, is uncovered by construction and is not separately enumerated.

Poll overlay_version/overlay_hash the same way as the tz_overlay_version/tz_overlay_hash fields on /timezone/offset — they move together.