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-cook-membrane/README.md

@@ -0,0 +1,7 @@
+# Cook's membrane at finite strain (pending)
+
+Waits for its reference value: the finite-strain Cook's membrane is defined (load, μ, κ) and
+answered in papers we have not transcribed, and no value is entered from memory. FieldCraft
+solves the model today (hybrid hex20 one element thick in plane strain, or plane strain quad8 for
+compressible materials); the case becomes active when the published numbers are copied in with
+their table.