The inner/outer pair was read from a file, rendered and written back, but nothing
could turn it on — a book layout could only be inherited. Both margin UIs get a
checkbox beside the four fields, and while it is on the left/right fields read
Inner/Outer. A margin preset changes the values, not whether the setup is a book.
`withMirrored` keeps the key absent rather than false, so an unmirrored page setup
still compares equal. Driven headlessly: ticking it stores the flag, renames the
fields and moves an even page's text by the difference (1.5/3cm → 57px).