inclusive-period-boundaries-and-offset-reset

Pick one boundary convention for a stepping period navigator and hold it everywhere — a closed upper bound at 23:59:59 drops the last second's sub-second remainder, and half-open bounds handed to an inclusive BETWEEN double-count the shared instant — then reset the step offset whenever the granularity changes, because N periods back at one length is not N periods back at another. Covers clamping at the present,…