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-hertz-cylinder/README.md

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+# Hertz line contact: cylinder on a flat (implicit mortar contact)
+
+A steel cylinder of radius R = 10 mm pressed on a steel block (E = 210 GPa, ν = 0.3; mm, N, MPa)
+in plane strain: a quarter model of the cylinder's lower part (a 2.5 mm wide block whose bottom
+is the circular arc, graded towards the contact) on a 5 × 5 mm foundation, symmetric about
+x = 0, one element thick with every node held in z. A pressure on the indenter's top carries
+the load P = 2265.57 N/mm (per length of the whole cylinder) chosen so that Hertz's half-width is
+b = 0.5 mm and the peak pressure p₀ = 2884.6 MPa. Soft springs (0.01 N/mm per node) hold the
+indenter's top vertically until the contact closes. One nonlinear static step without nonlinear
+geometry: small sliding, mortar contact of the round face (secondary) on the flat (main) by an
+augmented Lagrangian.