verification: Coulomb friction between sliding blocks

An oracle case of a block pressed on another and slid with static and kinetic coefficients
(mu_k N carried once slipping), and the docs of the friction formulation; ROADMAP M7 and
parity updated.

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

tests/oracle/nl-friction-sliding-blocks/README.md

@@ -0,0 +1,7 @@
+# Coulomb friction: a block slid on another (implicit mortar contact)
+
+A unit cube (2 × 2 × 2 hex8) on a 2 × 1 × 1 block (6 × 3 × 2 hex8), meshes not matching, E = 1000,
+ν = 0.3. The base of the lower block is held; the top of the upper block is pressed down by 0.02
+and moved 0.1 sideways in proportion over one nonlinear static step without nonlinear geometry.
+The contact pair has μ_s = 0.3 and μ_k = 0.2: the sideways demand far exceeds the friction, so
+the interface slips, with the kinetic coefficient once the nodes slip, and carries μ_k N.