On-Call Coverage Audit

Check that an on-call rota covers a period with no gap. Coverage is the union of the shifts in a rotation, so a gap is reported only where no shift covers the moment at all, which no per-shift check can find. Also reports one person on two rotations at once, where a page to both reaches one person so the second rotation is not a second responder; a shift longer than a stated maximum; too little rest between shifts; and a time with no UTC offset, which leaves a gap at every handover.