The architecture is verified at level 1: three arms sum to zero, the centre is
free. Everything above level 1 rests on one question — when you nest 27 of
these, does the centre stay free, or does a residual accumulate per level?
This page is the attempt to break it. It runs the nesting in exact integers
and in float64, side by side, in your browser, and reports where each one
fails. Nothing is fetched and nothing is stored.