Support rota coverage gaps

A team publishes its overnight support rota as a list of shifts, each written as a pair [start, end] in minutes counted from midnight. A shift covers every minute from start up to but not including end. The shifts arrive in no particular order, two of them can cover the same minute, a cancelled shift can carry start equal to end, and a shift that began the previous evening can carry a negative start. The rota is meant to cover the window running from minute 0 up to but not including day_end. Clip each shift to that window, then return every stretch inside the window with nobody on duty, each written as a pair [begin, finish] read the same way, sorted by begin. Return an empty list when the window is covered end to end, and also when day_end is zero or negative.