The body margin reserved the zone's height *plus* its fo:margin-top/-bottom, so
every page with a footer lost a line — a fixture ran a page over. Probed against
soffice: moving a footer's fo:margin-top leaves the last body line exactly where
it was and only shifts the footer text down its band; style:dynamic-spacing
changes neither side. The band is the declared height, the gap sits in it.
One fixture paginates to LibreOffice's 63 pages again.