presence.cycle_week_index: ((target_date - anchor_date).days // 7) %
cycle_length_weeks — anchor-date arithmetic ONLY (D-004). Python
floor division makes pre-anchor dates walk the cycle backwards
(day -1 is the last week), never inverting; the modulo of a negative
stays non-negative, so the index is always a valid week index. The
target date passes through the model's strict plain-date parsing.
Tests: formula offsets, three-week pattern wrap, backwards walking,
day-by-day stability across three Dec-31 -> Jan-1 boundaries
including 2020 (verified 53 ISO weeks — the parity-inversion
regression), and a source guard that the module never reads ISO week
numbers. Suite: 980 passed.
Joshua King
5885d5e972