solver: mortar contact in 2D models

Plane strain, plane stress and axisymmetric models take contact pairs and
general (self) contact on the linear edges of tri3 and quad4 elements:
edges oriented out of their elements, the edge mortar of fem with the
thickness or 2 pi r in the weights, the element's area over the edge's
length as the thickness under it, and the in-plane rows of the tangent
blocks. Verified by the contact patch test in plane strain and
axisymmetric (to 1e-9 small sliding, 1e-4 finite), Hertz's line contact
on quad4 edges (Johnson 1985, 4.3; equal to the hex8 model's to 1e-5),
and a hybrid rubber C-ring closing on itself that moves node for node as
the one-layer solid ring.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

tests/oracle/nl-contact-patch-axisymmetric/README.md

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+# Contact patch test, axisymmetric (implicit mortar contact, non-matching edges)
+
+The axisymmetric form of `nl-contact-patch`: two rings of unit square section, 1 ≤ r ≤ 2, stacked
+along the axis y and meshed independently (3 × 2 quad4 below, graded in r; 2 × 2 above), so the
+nodes of the interface at y = 1 do not match. A frictionless contact pair joins the lower ring's
+top edge (main) and the upper ring's bottom edge (secondary). The base y = 0 is held in y and
+the top y = 2 pushed down by 0.02 (E = 1000, ν = 0.3), in a `Small` (small sliding) and a `Finite`
+(finite sliding) nonlinear static step. Axial compression with free radial expansion is a
+uniform uniaxial stress σ_yy in both rings (u_r = −ν ε r, so the hoop strain equals the radial
+one), carried by a uniform contact pressure; the edge mortar integrals weight by 2πr, so the
+patch test checks that the radius enters the gaps and the forces consistently.