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/kirsch-plate/README.md

@@ -0,0 +1,7 @@
+# Plate with a circular hole in tension (Kirsch)
+
+A quarter of a square plate of half-width 30 with a hole of radius 1 (W = 60a, so the infinite
+plate's solution applies to a few tenths of a percent), thin (free faces, plane stress), with
+symmetry on x = 0, y = 0 and the mid-plane, pulled by a traction of 1 on x = 30. Two hex20 blocks
+around the hole (40 × 16 each), graded 40 : 1 towards it. Kirsch: σ_θ = 3S at the top of
+the hole, −S at its side.