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

crates/engine/src/cmd/contact.rs

@@ -1,7 +1,8 @@
 //! `interaction.contact.*`: contact definitions (pairs of surfaces, general contact) and the
 //! search for surfaces that touch; `interaction.tie*`: ties between non-matching meshes.
 //!
-//! A contact's surfaces are sets of element faces ([`fieldcraft_model::contact`]). The commands
+//! A contact's surfaces are sets of element faces, or of element edges in a 2D model
+//! ([`fieldcraft_model::contact`]). The commands
 //! take the friction, penalty scale and initial-penetration choice of the model's form; `edit`
 //! replaces a definition with the model's JSON form, `delete` removes it.
 //!