Ten documents the docx lib authors directly, plus the ODT twin
LibreOffice converts each into, now live in tests/corpus/ instead of the
gitignored fixtures dir. corpus.test.ts asserts both formats of a
document read the same and that each survives our export and import
either way. The heading styles gained the outline level Word writes —
without it LibreOffice converts a heading to a paragraph that merely
carries the style, and the two formats disagreed.