utils/chapterField owns the per-zone reading (header = the chapter at
the page top, footer = the last begun) so the probed LibreOffice
semantics is unit-tested; the DOM measurement feeding atTop stays in
Editor.svelte.
Word resolves the quoted style name against the localized built-in
name, so 'STYLEREF "Heading 1"' shows a field error in any
non-English Word. The numeric argument means outline level in Word
and LibreOffice alike (probed), and the importer already reads it.
A manual's footer carries text:chapter, which we imported as the string the
producer happened to cache — so every page of a fixture claimed the last chapter
of the book. It is now a live field: the layer resolves the last heading at
or above its outline level to have started by that page. Round-trips to ODF
text:chapter and Word STYLEREF; PDF keeps the cached name (Chrome's print
engine has no named strings).