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-patch-hex8/fixture.json

@@ -0,0 +1,41 @@
+{
+  "note": "FieldCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "sxx": {
+      "band": 4.2296179707574264e-8,
+      "value": 42.29617970757426
+    },
+    "sxy": {
+      "band": 8.89712243927903e-10,
+      "value": -0.8897122439279029
+    },
+    "sxz": {
+      "band": 1.8906385180297377e-9,
+      "value": -1.8906385180297376
+    },
+    "syy": {
+      "band": 2.3501008558807953e-8,
+      "value": 23.501008558807953
+    },
+    "syz": {
+      "band": 1.9462455331453503e-9,
+      "value": 1.9462455329577204
+    },
+    "szz": {
+      "band": 3.14528117370872e-8,
+      "value": 31.452811737087195
+    },
+    "ux": {
+      "band": 1.3583340405308903e-10,
+      "value": 0.13583340405308902
+    },
+    "uy": {
+      "band": 3.4874913736454575e-13,
+      "value": 0.0003487491350470007
+    },
+    "uz": {
+      "band": 2.3985080233668645e-12,
+      "value": -0.002398508023366864
+    }
+  }
+}