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.
