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/cdt-stepped-block/expected.json

@@ -0,0 +1,46 @@
+{
+ "title": "Constrained Delaunay mesh of a stepped block: exact volume, valid elements",
+ "kind": "theory",
+ "source": "mesh invariants: a tetrahedralization of a polyhedron fills its volume exactly (Si, TetGen paper ACM TOMS 41(2) 2015 for the definition of a CDT; Diazzi et al., ACM TOG 42(6) 2023); the volume is 60·40·10 + 40·40·10 + 20·40·10 − 10·10·10",
+ "quantities": [
+  {
+   "id": "volume",
+   "quantity": "volume of the tetrahedra",
+   "measure": {
+    "from": "tet",
+    "pointer": "/volume"
+   },
+   "value": 47000.0,
+   "tolerance": {
+    "relative": 1e-12
+   },
+   "reason": "a planar boundary is tessellated exactly and the constrained Delaunay mesh fills it exactly: only rounding"
+  },
+  {
+   "id": "invalid",
+   "quantity": "elements that fail the certified Jacobian check",
+   "measure": {
+    "command": "mesh.quality",
+    "pointer": "/invalid/count"
+   },
+   "value": 0.0,
+   "tolerance": {
+    "exact": true
+   },
+   "reason": "every element of a valid mesh has positive volume"
+  },
+  {
+   "id": "undetermined",
+   "quantity": "elements the Jacobian check could not decide",
+   "measure": {
+    "command": "mesh.quality",
+    "pointer": "/undetermined/count"
+   },
+   "value": 0.0,
+   "tolerance": {
+    "exact": true
+   },
+   "reason": "a linear tetrahedron's sign is exact"
+  }
+ ]
+}