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/fixture.json

@@ -0,0 +1,21 @@
+{
+  "note": "FieldCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "ux": {
+      "band": 1.2149180525904365e-11,
+      "value": -0.011149180525904363
+    },
+    "uy": {
+      "band": 7.959051161172887e-11,
+      "value": 0.07859051161172881
+    },
+    "uz": {
+      "band": 6.84413310858248e-11,
+      "value": -0.0674413310858248
+    },
+    "von_mises": {
+      "band": 1e-8,
+      "value": 4.868377314671631e-13
+    }
+  }
+}