Fill a BIN range with card brand intervals

A six-digit BIN covers every 16-digit card number that starts with it; the other ten digits run from 0000000000 to 9999999999. You get a list of inclusive suffix intervals, each labelled with a card brand. To hide exactly where each brand's range starts and ends, stretch the intervals until together they cover the whole BIN range, and return them as full 16-digit numbers sorted by start.

1. The intervals are contiguous: extend the first down to the start of the range and the last up to its end.
2. There may be gaps between intervals: give each gap to the interval below it.
3. Some intervals sit inside others: leave the nested ones unchanged and stretch only the outer ones.
4. Merge neighbouring intervals of the same brand into one.