Merge Booking Windows

A room booking system stores reservations as `(start, end)` minute offsets from midnight. Overlapping and touching reservations should be collapsed into single continuous blocks before they are shown on a calendar. Implement `merge_windows(windows)`, which returns the merged blocks in ascending order of start time. Treat the windows as **half-open** — `[60, 90)` and `[90, 120)` are adjacent and *do* merge into `[60, 120)`, because the room is never actually free at minute 90. Say so out loud in an interview: the convention is the part being assessed, not the loop.