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/plastic-thick-cylinder/README.md

@@ -1,3 +1,14 @@
-# Elastic–perfectly plastic thick cylinder: collapse pressure
+# Elastic–perfectly plastic thick cylinder: plastic zone and collapse pressure
 
-The Lamé cylinder of `lame-cylinder`, elastic–perfectly plastic, loaded to collapse.
+The Lamé cylinder of `lame-cylinder-axisymmetric` (a = 10, b = 20, an axisymmetric slice held
+axially on both faces, so plane strain), now elastic–perfectly plastic: E = 200000,
+ν = 0.4999 (Hill's solution takes the elasticity incompressible), σ_y = 250, one row of 20
+reduced quad8 elements through the wall, in two nonlinear static steps without nonlinear
+geometry:
+
+- **Pressure**: the bore pressure p = k (2 ln(c/a) + 1 − c²/b²) = 180.195 (k = σ_y/√3) that
+  takes the plastic zone to c = 15. The bore then moves u_a = k c²/(2 G a) = 0.024355: the plastic
+  zone moves incompressibly (u r constant) and the elastic ring outside yields at its inner edge.
+- **Collapse**: the bore pushed out by 0.3 (24 times its first-yield displacement), past the
+  pressure at which the wall is plastic through; the radial force on the bore stays at the
+  collapse pressure 2k ln(b/a) = 200.094 times 2πa per unit height.