BELFEM input-file topology section, generated from the actual physical
tags of the final mesh (see tmp/input.conf for the hand-written sample).
Curves are defined for BELFEM as sideset intersections (plane @ tape):
odd ids on the back plane, even ids on the front plane, per tape — the
same order as the sample. The periodic block names the vertex triples
(type-15 elements 1..10): three front quarter points (2, 3, 4) map to the
matching back quarter points (7, 8, 9); BELFEM derives the affine cap-to-cap
map from them. Node 1 (front cap center) is not part of the triple: three
points already fix that map, and the cap center lies on the axis.
The boundary conditions section drives current through the tapes: the odd
(back-plane) curves are the input curves, the even (front-plane) curves are
the output curves.