verification: implicit plasticity oracle cases and documentation

Four cases, each against its closed form: uniaxial tension of a J2 cube to a
logarithmic strain of 0.2 (Kirchhoff stress exact; 0.13 % from the explicit
solver's hypoelastic Cauchy stress, the elastic volume change between the two
laws), the Bauschinger effect of linear kinematic hardening on a load path
back to zero strain (exact), Hill's elastic-perfectly plastic thick cylinder
(bore displacement at a plastic radius of 1.5a within 0.009 %, collapse
pressure within 2e-8; the pending case is now active) and a cantilever's
plastic collapse, converged 3.1 % above beam theory's hinge, the bound a
continuum exceeds. docs/fem.md, the literature, ROADMAP (M15), parity and the
oracle page follow.

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

tests/oracle/nl-j2-uniaxial/README.md

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+# Uniaxial tension of a J2 bilinear-hardening cube to large strain (implicit)
+
+The cube of `j2-uniaxial-explicit` solved by the implicit nonlinear solver: one fully integrated
+hex8, E = 200000, ν = 0.3, σ_y = 250, H = 2000 (isotropic), held by symmetry on three faces and
+stretched on the fourth to λ = e^0.2 in a nonlinear static step, lateral faces free.
+
+Simo's exponential return map integrates this proportional path exactly in the logarithmic
+strain, so the Kirchhoff stress is the small-strain curve τ = σ_y + E_t (ε − σ_y/E) = 643.564 at
+ε = 0.2 and the equivalent plastic strain (τ − σ_y)/H = 0.196782. The Cauchy stress is
+σ = τ/J with Hencky's volume law ln J = τ/(3K): 642.737.
+
+Cross-check with the explicit solver: `j2-uniaxial-explicit` reaches 643.57 for the Cauchy stress
+with the same plastic strain. Its hypoelastic update takes the logarithmic strain rate against
+the Cauchy stress, while the implicit model is hyperelastic in the Kirchhoff stress: the two
+laws differ by the factor J = 1.0013 (the elastic volume change), which is the 0.13 % between
+them; both meet the closed form of their own law.