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/cantilever-hex20/expected.json

@@ -0,0 +1,43 @@
+{
+ "title": "Cantilever under a tip load, Timoshenko deflection (hex20)",
+ "kind": "theory",
+ "source": "Timoshenko and Gere, Mechanics of Materials (beam with shear deformation: δ = PL³/3EI + PL/κGA, κ = 5/6 for a rectangle); Cowper, J. Appl. Mech. 33 (1966) 335–340 for the shear coefficient",
+ "quantities": [
+  {
+   "id": "tip_uz",
+   "quantity": "tip deflection u_z at (10, 0.5, 0.5)",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Bend",
+     "at": [
+      [
+       10.0,
+       0.5,
+       0.5
+      ]
+     ]
+    },
+    "pointer": "/nodes/0/displacement/2"
+   },
+   "value": -4.024,
+   "tolerance": {
+    "relative": 0.005
+   },
+   "reason": "beam theory vs 3D elasticity: the shear coefficient and the fully clamped end change the tip deflection by a few tenths of a percent at L/h = 10"
+  },
+  {
+   "id": "reaction",
+   "quantity": "total reaction z",
+   "measure": {
+    "from": "solve",
+    "pointer": "/reaction_total/2"
+   },
+   "value": 1.0,
+   "tolerance": {
+    "relative": 1e-09
+   },
+   "reason": "equilibrium holds to rounding"
+  }
+ ]
+}