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/j2-bauschinger/expected.json

@@ -0,0 +1,45 @@
+{
+ "title": "Bauschinger effect of linear kinematic hardening: tension, then back to zero strain",
+ "kind": "theory",
+ "source": "J2 plasticity with Prager's linear kinematic hardening in uniaxial stress (W. Prager, Ing.-Arch. 1955; Simo and Hughes, Computational Inelasticity, Springer 1998, §2.3): from σ_a = σ_y + E_t (ε_a − σ_y/E) the material unloads elastically by 2σ_y and then hardens with E_t, so at zero strain σ = σ_a − 2σ_y − E_t (ε_a − 2σ_y/E)",
+ "quantities": [
+  {
+   "id": "residual_stress",
+   "quantity": "axial stress back at zero strain after a pull to ε_a = 0.01",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Pull",
+     "elements": [
+      1
+     ]
+    },
+    "pointer": "/elements/0/stress/0/0"
+   },
+   "value": -247.52475247524754,
+   "tolerance": {
+    "relative": 1e-06
+   },
+   "reason": "a homogeneous small-strain state the element represents exactly, with increments that end at the load path's turn; isotropic hardening would give −281.835 (it yields again only at −σ_a)"
+  },
+  {
+   "id": "plastic_strain",
+   "quantity": "accumulated equivalent plastic strain",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Pull",
+     "elements": [
+      1
+     ]
+    },
+    "pointer": "/elements/0/plastic_strain/0"
+   },
+   "value": 0.016089108910891083,
+   "tolerance": {
+    "relative": 1e-06
+   },
+   "reason": "(σ_a − σ_y)/H forward plus E/(E + H) (ε_a − 2σ_y/E) in reverse"
+  }
+ ]
+}