verification: nonlinear oracle cases and the nonlinear solver's documentation

Theory cases for the nonlinear solver, each frozen as a fixture: uniaxial
stretch of a neo-Hookean block to twice its length (closed form), the
nonlinear patch test on a distorted hex8 mesh, a quarter turn of the same
mesh (objectivity: zero stress), the elastica of a hex20 cantilever at
P L²/(E I) = 1, 2, 5, 10 against Bisshopp–Drucker (0.2–0.5 %), and the
inflation of a thick neo-Hookean tube by a follower pressure against
Rivlin's closed form (0.08 % at the bore). A finite-strain Cook's membrane
is pending its transcribed reference. docs/fem.md describes the
formulation, the material point interface, the follower-pressure choice and
what nonlinear steps refuse; the literature, parity and roadmap pages
follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

tests/oracle/nl-rigid-rotation/README.md

@@ -0,0 +1,6 @@
+# Rigid rotation: objectivity
+
+The distorted 3 × 3 × 3 hex8 cube of nl-patch-hex8 (neo-Hookean, E = 1000, ν = 0.3) with every
+boundary node prescribed the motion of a 90° rotation about the axis (1, 1, 1)/√3, applied in
+one ramp (the intermediate states are not rigid; the end state is). The inner nodes must follow
+the rotation and the stress must vanish.