The naive formula used (jan4.Weekday() - time.Monday) produces -1
when Jan 4 is a Sunday (Weekday()==0), shifting the computed Monday
one week forward. 2026 is affected: Jan 4 is a Sunday, so every
week key in 2026 was mapped to the wrong 7-day range, causing
CloseWeek to look for closed_days on the wrong dates and finding
nothing — resulting in worked_ms=0 and a full -Nh delta.
Fix: use (weekday+6)%7 to get days-since-Monday (Mon=0…Sun=6),
which is always non-negative.
Adds table-driven TestWeekDayKeys covering 2024 (Thu), 2026 (Sun),
and 2023 (Wed) to prevent regression.
Andreas Schneider
e37458f513