2D elements: plane stress, plane strain and axisymmetric sections on the DOF framework; mesh.planar (M4) (#39)

Model: planar section kinds with Extent, element edges in sets, edge-weighted 2D loads,
centrifugal load, 2D model checks. fem: Plane kernels on Basis::{Tri3,Tri6,Quad4,Quad8}
declaring ElementDofs::new(DofSet::PLANE); QM6 incompatible modes with the
Wilson-Ibrahimbegovic correction. Solver: Kernel enum on the DofLayout; 3D results
bit-identical (hash-checked). Decks: Abaqus CPS/CPE/CAX, Nastran membranes and RFORCE,
LS-DYNA ELFORM 12/13. Mesh: free triangle mesher, planar region meshing, face boundaries.
Engine: mesh.planar with tagged edges. Analytic verification suite in solver/tests/plane.rs.


Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/fem/src/lib.rs

@@ -15,6 +15,8 @@
 //! - [`material`]: isotropic and orthotropic elasticity matrices, von Mises stress;
 //! - [`element`]: the element table, stiffness (full, reduced, incompatible modes), consistent
 //!   and lumped mass, body forces, integration-point stresses and their extrapolation to nodes;
+//! - [`plane`]: 2D elements (plane stress, plane strain, axisymmetric) on tri3, tri6, quad4 and
+//!   quad8, with two displacement components per node;
 //! - [`face`]: consistent nodal forces of pressure on element faces;
 //! - [`uniform`]: uniform-strain (one-point) hexahedron and tetrahedron kinematics, hourglass
 //!   shape vectors and element frequency bounds for explicit dynamics;
@@ -35,6 +37,7 @@ pub mod element;
 pub mod face;
 pub mod jet;
 pub mod material;
+pub mod plane;
 pub mod quadrature;
 pub mod shape;
 pub mod uniform;