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

crates/testkit/src/lib.rs

@@ -4,8 +4,12 @@
 //! `tests/corpus/<name>/recipe.json`, and analytic shapes generated in code with closed-form
 //! volumes. Meshing tests load their geometry from here; [`runner`] (the `corpus` binary,
 //! `cargo xtask corpus`) builds and meshes every part and writes docs/meshing/corpus.md.
+//!
+//! [`oracle`] (the `oracle` binary, `cargo xtask oracle`) runs the verification and validation
+//! cases of `tests/oracle` against their references and fixtures and writes docs/oracle.md.
 #![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
 #![forbid(unsafe_code)]
 
 pub mod corpus;
+pub mod oracle;
 pub mod runner;