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-plane-strain/README.md

@@ -0,0 +1,10 @@
+# Contact patch test in plane strain (implicit mortar contact, non-matching edges)
+
+The 2D form of `nl-contact-patch`: two unit squares stacked along y in plane strain (2 mm thick),
+meshed independently: 3 × 2 quad4 below (graded in x), 2 × 2 above, so the nodes of the interface
+at y = 1 do not match. A frictionless contact pair joins the lower block's top edge (main) and
+the upper block's bottom edge (secondary). The edge x = 0 is a symmetry line, the base y = 0 is
+held in y, and the top y = 2 is pushed down by 0.02 (E = 1000, ν = 0.3) in two nonlinear static
+steps: `Small` without nonlinear geometry (small sliding) and `Finite` with it (finite sliding).
+The exact state is uniaxial stress σ_yy (σ_zz = ν σ_yy out of plane), uniform in both blocks,
+with a uniform contact pressure equal to it.