oracle: 2D cases (Kirsch plane stress, Lamé plane strain and axisymmetric, rotating disc and cylinder, NAFEMS LE1 with quad8) and J2 uniaxial explicit

The pending 2D LE1 case becomes a real one; the plasticity cases stay
pending with what they still need (static J2 for the collapse pressure,
the transcribed measurements for the Taylor bar).

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

tests/oracle/rotating-disc-plane-stress/README.md

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+# Rotating disc with a central hole, plane stress
+
+A thin disc, inner radius 20, outer 100 (E = 210000, ν = 0.3, ρ = 7.85e-9: mm, t, s, MPa),
+spinning at 1000 rad/s about z: a quarter, one quad8 patch (16 × 32, graded 4 : 1 towards the
+bore, where the hoop stress is steepest) with mid-side nodes on the arcs, symmetry on x = 0 and y = 0, a centrifugal load. Timoshenko and Goodier:
+σ_r = (3 + ν)/8 ρω² (b² + a² − a²b²/r² − r²),
+σ_θ = (3 + ν)/8 ρω² (b² + a² + a²b²/r² − (1 + 3ν)/(3 + ν) r²).