Split blocks on a local-rhythm gap with a size change: 6724771 passes end-to-end

_lines_to_blocks judged every vertical gap against the page-median line
height, so it missed paragraph breaks inside tightly-set banners.
6724771 sets its affiliation legend at 11pt with 2.2px line spacing and
the abstract right beneath it at 9.6pt: the boundary is a 9.5px gap and
a 1.15x size change, which clears neither the page-median gap threshold
nor the 1.3x size-jump test. Legend and abstract fused into one block,
so the trailing legend entry swallowed the abstract and the parse was
discarded.

A gap over three times the rhythm the block has itself established,
combined with a font-size change, is now a break. Both conditions are
required and an established rhythm (three lines) is needed before the
rule can fire, so uniformly-set prose is never broken on gap alone. The
legend and abstract now separate exactly at the real boundary (block
ends 319.7, abstract starts 329.2).

6724771 LOGIC-GAP to PASS. AFFIL 10/13 numbered posters end-to-end,
18/21 acceptable. Zero per-poster metric change across the corpus, suite
green (260 passed).

Author: jimnoneill