Per user, find maximal runs where gaps of UP TO 1 missed day still count as continuous (so a sequence like day 1, day 2, day 4, day 5 is ONE tolerant run because day 3 is the only missed day between the two halves).
For each tolerant run, return its run_start, run_end, and the number of days_present (actual login rows in the run).
Sort by user_id then run_start ascending.