Explicit X2: large deformation, J2 and Johnson-Cook plasticity, damage and erosion (M10)

- model: materials gain solver-neutral plastic (bilinear isotropic/kinematic,
  hardening table, Johnson-Cook with adiabatic heating), damage (Johnson-Cook
  D1-D5) and erosion (plastic strain); explicit steps gain nonlinear_geometry
  (default on) and stress_rate (jaumann | green_naghdi).
- fem: rotation (Hughes-Winget, Flanagan-Taylor), plasticity (radial return,
  capped bracketed Newton for Johnson-Cook, damage; the update takes the
  mechanical strain increment so thermal strains can be subtracted), closed-form
  hex8 gradients for deformed configurations.
- explicit: updated Lagrangian hex8/tet4, per-element material state, erosion
  with reported energy, plastic work in the energy account.
- solver: warns that the linear solver uses the elastic constants of plastic
  materials (results hash-identical).
- decks: Abaqus *PLASTIC (+ Johnson-Cook keywords), Nastran MATS1/TABLES1,
  LS-DYNA *MAT_PLASTIC_KINEMATIC / *MAT_PIECEWISE_LINEAR_PLASTICITY; what a
  format cannot express is reported as Unsupported.
- engine: model.material.plasticity, model.material.damage (catalogue ready),
  explicit element probe, eroded elements in the solve summary.

Step commits before the rebase onto the DOF framework: branch explicit-x2-steps.

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

crates/fem/src/lib.rs

@@ -20,6 +20,10 @@
 //! - [`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;
+//! - [`rotation`]: incrementally objective stress updates (Hughes–Winget Jaumann, Flanagan–Taylor
+//!   Green–Naghdi);
+//! - [`plasticity`]: J2 radial return with isotropic, kinematic and Johnson–Cook hardening,
+//!   Johnson–Cook damage;
 //! - [`dense`]: the small dense linear algebra inside an element.
 //!
 //! Conventions: element matrices are ordered node by node, each node's degrees of freedom in
@@ -38,7 +42,9 @@ pub mod face;
 pub mod jet;
 pub mod material;
 pub mod plane;
+pub mod plasticity;
 pub mod quadrature;
+pub mod rotation;
 pub mod shape;
 pub mod uniform;