The edge→zone distance was document-wide, so an index whose header starts 4cm
down had its body open 14.5mm too high and the page under it followed. HfSet
carries the section's own pair (and its first page's, where the page style hands
over): ODF's page-layout margin is that distance, Word's is w:pgMar
w:header/w:footer, and both exports write it back.
A document had one set for all of it, taken from whichever section
declared one last — so a fixture lost its letterhead entirely
and page 1 got the empty header its final section asked for.
A body block now carries `sectionBreak`, ordinal rather than indexed so
editing cannot desync it. Import resolves Word's "Link to Previous" per
reference type; export writes a set per sectPr, and on the ODF side each
section past the first gets its own master page, which the block switches
to. The layer picks a page's set from where pageBreaks reports the
sections starting, and each section's zone reaches ride
`--pb-section-reach` so a later section's letterhead makes room for
itself: page 10 of a fixture was 24.9mm out, now 1.1mm.
Editing still reaches section 1 only; the rest render read-only.