End caps: meshed ONCE from exact planar geometry (built-in kernel
circles, transfinite arcs whose nodes land on the layer slot angles), then
instantiated rigidly at s=0 and, rotated by the common cap rotation theta,
at s=L. Cap periodicity holds node-for-node by construction.
Boundary nodes are snapped to their exact analytic positions after the
gmsh 2D run (gmsh transfinite placement is ~4e-9 off, Spike C) and REUSE
the ring node ids of the strips, so the shell is watertight with a single
global node id space.