A paragraph (or paragraph fragment) at a page boundary together with the
number of justified output lines it occupies when laid out alone. The
line_count lets a consumer splice a re-justified seam back into the
page’s lines by replacing exactly the partial’s worth of lines.
Re-justify a seam paragraph formed by joining the trailing partial of one
page with the leading partial of the next. The joined text is fed through
the standard PDF justifier so soft hyphens, mid-word breaks and similar
cross-line repairs happen as if the paragraph had never been split.