The docx package cannot express a custom footnote/endnote mark, so the anchor
always went out as an automatic reference and the label was lost — and the
importer never read one either, so a Word file's custom mark leaked into the
note's body text. A post-pack pass now rewrites the anchor (flag + literal
run) and the note part's <w:footnoteRef/>; the importer reads the flag, takes
the run's text as the label and strips its repeat off the note body.