fem: mortar segmentation of face pairs with forward-mode automatic differentiation

The overlap of a secondary face with a main face (projection onto the secondary plane,
Sutherland-Hodgman clipping of their triangles, Radon's seven-point rule on the overlap's
fan) and the inverse linear and bilinear maps of both faces, written over a Real scalar so
that f64, Dual (gradient) and Hyper (gradient and packed Hessian, 24 variables) give the
values and the exact linearisation of the segmentation, the normal and the projection.

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

crates/fem/src/lib.rs

@@ -30,6 +30,8 @@
 //! - [`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;
+//! - [`mortar`]: mortar segmentation of a secondary face against a main face and the integration
+//!   points on the overlap, over [`dual`]'s automatic differentiation for the exact tangent;
 //! - [`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
@@ -59,6 +61,7 @@ pub mod constitutive;
 pub mod contact;
 pub mod dense;
 pub mod dof;
+pub mod dual;
 pub mod elastoplastic;
 pub mod element;
 pub mod face;
@@ -68,6 +71,7 @@ pub mod heat;
 pub mod hybrid;
 pub mod jet;
 pub mod material;
+pub mod mortar;
 pub mod plane;
 pub mod plasticity;
 pub mod quadrature;