alignment_frame measured a one-row paragraph's alignment in the frame
of the nearest overlapping block. A full-width heading closer to the
title than the title's own body column gave it a page-wide frame, so
centring or right-aligning the title moved it out of its column. It now
considers every overlapping block whose column holds the row and takes
the narrowest, the nearest on a tie, so a page-wide heading gives the
title no frame while a narrower column that holds it does. When no
column holds the row it is still measured in its own box.
The comment about page-wide headers named an exclusion the filter
never made; the doc comment now says what is chosen and why.