verification: hyperelastic and thermal oracle cases and documentation

Five cases against closed forms: a Mooney-Rivlin block in Treloar's
uniaxial, equibiaxial and pure-shear modes and a Yeoh block in uniaxial
tension (the compressible energy's stresses, exact); a stretched
Mooney-Rivlin tube inflated by a follower pressure (Rivlin's incompressible
solution, 0.6 % at nu = 0.4995); free and fully held thermal expansion at a
thermal stretch of 1.2 (exact); a bar held at its ends heated past yield
(exact). docs/fem.md, the literature, ROADMAP (M15, M17), 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-mooney-rivlin-modes/README.md

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+# Mooney–Rivlin block in Treloar's three modes
+
+One hybrid hex20 unit cube on three symmetry planes, E = 3, ν = 0.49 (μ = 1.006711,
+κ = 50), Mooney–Rivlin with C₀₁ = 0.2 (C₁₀ = μ/2 − C₀₁ = 0.303356), in three nonlinear
+static steps:
+
+- **Uniaxial**: the face x = 1 pulled to λ = 2, the others free.
+- **Equibiaxial**: x = 1 and y = 1 pulled to λ = 1.5, z free.
+- **Pure shear**: x = 1 pulled to λ = 2, y = 1 held (λ_y = 1), z free.
+
+The references are the compressible decoupled energy's principal Cauchy stresses
+σᵢ = (λᵢ/J) ∂W/∂λᵢ, by differences of W alone, with the free stretch found by bisection so the
+free faces carry no stress. The deformation is homogeneous, so the element represents it
+exactly.