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/mh-solid-patch/README.md

@@ -0,0 +1,6 @@
+# MacNeal–Harder patch test for solids (hex8 with incompatible modes)
+
+Our model of the published problem: a unit cube cut into seven hexahedra around an irregular inner
+hexahedron (inner node coordinates from Fig. 3), E = 10⁶, ν = 0.25, with the linear displacement
+field of Table 2(c) prescribed at the eight corners. The published solution is the uniform strain
+10⁻³ with σ_x = σ_y = σ_z = 2000 and τ = 400.