Verification oracle: tests/oracle cases, cargo xtask oracle, fixtures, docs/oracle.md, CI job

The oracle binary (modelcraft-testkit) runs each case's recipe through the
command engine, measures its quantities by JSON pointers into step outputs
or commands, compares them with the cited references and the frozen
fixtures, and writes docs/oracle.md. 20 theory cases (patch tests of every
solid element, Timoshenko cantilevers, Lame, Kirsch, beam frequencies,
torsion and bending through couplings, explicit waves and impact, CDT
invariants), 8 published benchmarks (MacNeal-Harder patch, straight and
curved cantilevers, thick cylinder, Scordelis-Lo with solids, NAFEMS LE1,
LE10, FV32) and 10 pending cases with citations.

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

tests/oracle/nafems-le1/expected.json

@@ -0,0 +1,30 @@
+{
+ "title": "NAFEMS LE1 elliptic membrane (hex20 plane stress layer)",
+ "kind": "benchmark",
+ "source": "NAFEMS, The Standard NAFEMS Benchmarks, TNSB Rev. 3, October 1990, test LE1 (target σ_yy = 92.7 MPa at D); as reproduced in the Abaqus 2017 Benchmarks Guide, 'LE1: Plane stress elements—elliptic membrane'",
+ "quantities": [
+  {
+   "id": "syy_D",
+   "quantity": "σ_yy at D (2, 0)",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Load",
+     "at": [
+      [
+       2,
+       0,
+       0
+      ]
+     ]
+    },
+    "pointer": "/nodes/0/stress/1"
+   },
+   "value": 92.7,
+   "tolerance": {
+    "relative": 0.01
+   },
+   "reason": "the target is published to three figures; plane stress modelled by a thin solid layer and the stress extrapolated to the edge add a few tenths of a percent"
+  }
+ ]
+}