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/lame-cylinder/expected.json

@@ -0,0 +1,76 @@
+{
+ "title": "Thick-walled cylinder under internal pressure, plane strain (Lamé)",
+ "kind": "theory",
+ "source": "Lamé's solution, Timoshenko and Goodier, Theory of Elasticity, 3rd ed., McGraw-Hill 1970, §28 (thick-walled cylinder)",
+ "quantities": [
+  {
+   "id": "ur_a",
+   "quantity": "radial displacement at the bore (10, 0, 0)",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Pressure",
+     "at": [
+      [
+       10.0,
+       0,
+       0
+      ]
+     ]
+    },
+    "pointer": "/nodes/0/displacement/0"
+   },
+   "value": 0.00907936507936508,
+   "tolerance": {
+    "relative": 0.001
+   },
+   "reason": "quadratic elements on 8 radial divisions resolve u_r = A r + B/r to well under 0.1%"
+  },
+  {
+   "id": "ur_b",
+   "quantity": "radial displacement outside (20, 0, 0)",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Pressure",
+     "at": [
+      [
+       20.0,
+       0,
+       0
+      ]
+     ]
+    },
+    "pointer": "/nodes/0/displacement/0"
+   },
+   "value": 0.005777777777777778,
+   "tolerance": {
+    "relative": 0.001
+   },
+   "reason": "as for the bore"
+  },
+  {
+   "id": "hoop_a",
+   "quantity": "hoop stress σ_θ = σ_yy at the bore (10, 0, 0)",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Pressure",
+     "at": [
+      [
+       10.0,
+       0,
+       0
+      ]
+     ]
+    },
+    "pointer": "/nodes/0/stress/1"
+   },
+   "value": 166.66666666666666,
+   "tolerance": {
+    "relative": 0.01
+   },
+   "reason": "stress extrapolated from the integration points to the bore surface and averaged over two elements: about 0.5% at the steepest gradient"
+  }
+ ]
+}