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/patch-hex8/README.md

@@ -0,0 +1,8 @@
+# Constant-stress patch test, hex8
+
+A unit cube of eight hex8 cells whose shared points are moved off the regular grid (every
+face, edge and centre point; the corners stay), so every element is distorted (one mesh.block per cell). Symmetry on the
+faces x = 0, y = 0 and z = 0; tractions 1, 2 and 3 on the faces x = 1, y = 1 and z = 1
+(E = 1000, ν = 0.25). The exact solution is the uniform stress diag(1, 2, 3) with strains
+ε_i = (σ_i − ν(σ_j + σ_k))/E, which every conforming element of a convergent formulation
+reproduces exactly on any mesh.