verification: implicit contact patch test and Hertz line contact

Two oracle cases: the contact patch test on non-matching meshes in small and finite sliding,
and Hertz's cylinder on a flat under force control (peak pressure, pressure distribution,
subsurface stress). docs/fem.md describes the formulation, the literature survey its papers;
ROADMAP M7 and parity are updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

tests/oracle/nl-contact-patch/README.md

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+# Contact patch test (implicit mortar contact, non-matching meshes)
+
+Two unit cubes stacked along z, meshed independently: 3 × 3 × 2 hex8 below (graded in x and y),
+2 × 2 × 2 above, so the nodes of the interface at z = 1 do not match. A frictionless contact pair
+joins the lower block's top (main) and the upper block's bottom (secondary). The faces x = 0 and
+y = 0 of both blocks are symmetry planes, the base z = 0 is held in z, and the top z = 2 is pushed
+down by 0.02 (E = 1000, ν = 0.3) in two nonlinear static steps: `Small` without nonlinear
+geometry (small sliding, mortar integrals in the reference configuration) and `Finite` with it
+(finite sliding, searched and integrated on the deformed faces). The exact state is uniaxial
+stress, uniform in both blocks, with a uniform contact pressure equal to it.