The sparse elliptic-cone Hessian assembly (JTCJ) previously ran
output-stationary: one thread per (contact, dense dof-pair), scanning
the contact's column indices to locate each pair. When nefc >> nv the
overwhelming majority of those dof-pairs do not appear in the contact's
support, so the kernel spent almost all of its time scanning and
skipping.

Restructure it to be input-stationary: launch one thread per
(contact, support-pair), decode the pair index directly into the two
participating dofs, register-sum the cone block, and accumulate into H
with a single atomic add. This touches only the pairs that actually
contribute and exposes far more parallelism. The pair dimension is
bounded by jtcj_max_pairs, derived once in io.py from the deepest
geom-body dof chain. The math is unchanged.