Start of the billing month: the most recent occurrence of the inferred
anniversary day, else the first of the calendar month. Returns (start, inferred).
Which of this model’s windows is over its share of the monthly limit, if
any — the verdict the sidebar’s flag, the watch frame and the cap
notification all read from. Go meters per model AND per window (5-hour =
20%, weekly = 50%, monthly = 100%), so any one of them counts; the shortest
window wins, being the most actionable. None for a free/unlimited model.