The page grid was one repeating cycle, so every page had to be the same
size. It is now a table of height runs the placement pass grows as each
section's first page becomes known, and the sheets are one box per page
instead of a repeating gradient — .paper reserves the widest section's
paper, a narrower one takes the difference as a right inset.
Layout ▸ Orientation offers "this section"; the size stays document-wide
in the ribbon but round-trips per section from a file.