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/pinched-cylinder/expected.json

@@ -0,0 +1,14 @@
+{
+ "title": "Pinched cylinder with rigid diaphragms",
+ "kind": "benchmark",
+ "source": "Belytschko, Stolarski, Liu, Carpenter and Ong, 'Stress projection for membrane and shear locking in shell finite elements', Computer Methods in Applied Mechanics and Engineering 51 (1985) 221–258 (the shell obstacle course); reference 1.8248 × 10⁻⁵ under the load",
+ "status": "pending",
+ "requires": "shell elements (M8)",
+ "quantities": [
+  {
+   "id": "u",
+   "quantity": "radial displacement under the load",
+   "value": -1.8248e-05
+  }
+ ]
+}