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-frequencies/expected.json

@@ -0,0 +1,46 @@
+{
+ "title": "Natural frequencies of a cantilever (Euler–Bernoulli)",
+ "kind": "theory",
+ "source": "Euler–Bernoulli beam: f_n = (β_n L)²/(2π L²)·√(EI/ρA), β₁L = 1.8751, β₂L = 4.6941; Timoshenko, Young and Weaver, Vibration Problems in Engineering, 4th ed., Wiley 1974, §5.3",
+ "quantities": [
+  {
+   "id": "f1",
+   "quantity": "first bending frequency (modes 1 and 2 are the y and z pair)",
+   "measure": {
+    "from": "modes",
+    "pointer": "/modes/0/frequency"
+   },
+   "value": 0.012770863695053732,
+   "tolerance": {
+    "relative": 0.01
+   },
+   "reason": "Euler–Bernoulli neglects shear and rotary inertia, which lower f1 by about 0.2% at L/h = 20"
+  },
+  {
+   "id": "f1_pair",
+   "quantity": "the other first bending frequency",
+   "measure": {
+    "from": "modes",
+    "pointer": "/modes/1/frequency"
+   },
+   "value": 0.012770863695053732,
+   "tolerance": {
+    "relative": 0.01
+   },
+   "reason": "as f1: a square section bends alike in y and z"
+  },
+  {
+   "id": "f2",
+   "quantity": "second bending frequency",
+   "measure": {
+    "from": "modes",
+    "pointer": "/modes/2/frequency"
+   },
+   "value": 0.08003363662360005,
+   "tolerance": {
+    "relative": 0.02
+   },
+   "reason": "shear and rotary inertia lower the second mode by about 1% at L/h = 20 (Timoshenko)"
+  }
+ ]
+}