fem: J2 plasticity at the implicit material point, finite and small strain

The radial return of the explicit update is now J2::radial_return, called by
both solvers, with the consistent tangent J2::tangent (Simo & Hughes 1998,
Box 3.2). The new elastoplastic module gives the implicit material point:
Simo's multiplicative plasticity with the exponential return map in the
principal logarithmic strains (CMAME 99, 1992) and its spatial tangent through
the Daleckii-Krein divided differences of the logarithm, and the additive
radial return of small strain with kinematic hardening. MaterialModel::Plastic
carries it, decoupled (deviatoric return, Hencky volume) so hybrid elements
take it. The element loops of finite.rs gain small-strain kinematics for
inelastic materials and pass history through the mixed path. Every tangent is
checked against central differences, element by element, from a plastic
history.

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

crates/solver/tests/nonlinear.rs

@@ -130,7 +130,7 @@ fn nonlinear_patch_test() {
     };
     let (e, nu) = (100.0, 0.3);
     let neo = fieldcraft_fem::constitutive::MaterialModel::from_model(&iso(e, nu), Some(&Hyperelastic::NeoHookean {})).unwrap();
-    let tau = neo.response(&f, &[], &mut []).unwrap().stress;
+    let tau = neo.response(&f, None, &[], &mut []).unwrap().stress;
     let j = fieldcraft_fem::dense::det3(&f);
     let hybrid = Formulation { hybrid: true, ..Formulation::default() };
     let mut cases: Vec<(ElementKind, Formulation, &str)> = variants().into_iter().filter(|v| v.1 != incompatible()).collect();