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/plastic-thick-cylinder/expected.json

@@ -3,7 +3,7 @@
  "kind": "theory",
  "source": "Hill, The Mathematical Theory of Plasticity, Oxford 1950, ch. V §2: plane strain von Mises collapse pressure p = (2/√3)σ_y ln(b/a)",
  "status": "pending",
- "requires": "J2 plasticity (M8/M10 X2)",
+ "requires": "implicit (static) J2 plasticity: explicit J2 is in, a collapse load needs the static solver",
  "quantities": [
   {
    "id": "p_collapse",