model: contact pairs, general contact and Coulomb friction, mapped by the deck writers

Contact definitions on face sets: a pair (each surface's nodes against the
other's faces), general contact on a surface (self contact) or on every
exterior face of the solids; Coulomb friction with static and kinetic
coefficients and an exponential decay, a penalty scale and the choice to
adjust or refuse initially penetrating nodes, validated with caps. Rigid
walls of explicit steps gain friction. The check wants solid faces on
contact surfaces; sets used by a contact cannot be deleted.

fem::contact: the closest point of a triangle (Ericson) or a bilinear
quadrilateral (projected Newton) to a point, and a bucket grid of face
boxes for the global search, deterministic and bounded.

Commands interaction.contact.pair, .general, .edit, .delete and .find
(the touching parts of the mesh, with face sets and pairs made of them);
contacts in the model browser. Abaqus writes *SURFACE INTERACTION with
*FRICTION (exponential decay), *CONTACT PAIR and general *CONTACT; LS-DYNA
*CONTACT_AUTOMATIC_SURFACE_TO_SURFACE and _SINGLE_SURFACE with FS, FD, DC;
Nastran refuses contact by name. The implicit solver refuses contact by
name (M7); the explicit solver refuses it until the next commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/fem/src/lib.rs

@@ -28,6 +28,8 @@
 //! - [`beam`]: two-node Timoshenko beams with profile section properties;
 //! - [`frame`]: local axes of shells and beams and the rotation of their matrices;
 //! - [`face`]: consistent nodal forces of pressure on element faces;
+//! - [`contact`]: contact geometry: the closest point of an element face to a point and the
+//!   bucket grid of the global search;
 //! - [`heat`]: heat conduction (conductivity, capacity, sources, surface flux and convection,
 //!   heat flux); thermal strain loads and stresses are in [`element`];
 //! - [`uniform`]: uniform-strain (one-point) hexahedron and tetrahedron kinematics, hourglass
@@ -54,6 +56,7 @@
 pub mod basis;
 pub mod beam;
 pub mod constitutive;
+pub mod contact;
 pub mod dense;
 pub mod dof;
 pub mod elastoplastic;