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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

.github/workflows/ci.yml

@@ -88,6 +88,24 @@ jobs:
           save-if: ${{ github.ref == 'refs/heads/main' }}
       - run: cargo xtask test --heavy
 
+  oracle:
+    # Verification and validation (docs/verification.md): every case against its reference and
+    # its fixture, and docs/oracle.md must be the table the cases produce. Cases marked `slow`
+    # run here too while the whole set stays within a few minutes; `--quick` skips them.
+    name: cargo xtask oracle
+    runs-on: ubuntu-24.04
+    timeout-minutes: 45
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+        with:
+          key: oracle
+          save-if: ${{ github.ref == 'refs/heads/main' }}
+      - run: cargo xtask oracle
+      - name: docs/oracle.md is current
+        run: git diff --exit-code docs/oracle.md
+
   advisories:
     name: Dependency advisories and sources
     needs: changes