verification: a nearly incompressible hybrid block under prescribed stretch

nl-hybrid-incompressible: 27 hybrid hex8 at ν = 0.4999 pulled to λ = 2 by a prescribed
displacement, against the compressible neo-Hookean closed form (1.4·10⁻⁹); its first
iterations need the LDLᵀ factorization. ROADMAP and parity updated, docs/oracle.md regenerated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/oracle/nl-hybrid-incompressible/README.md

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+# Nearly incompressible hybrid block under prescribed stretch
+
+A unit cube of 3 × 3 × 3 hybrid hex8 elements on three symmetry planes, neo-Hookean with E = 3,
+ν = 0.4999 (κ/μ ≈ 5000), pulled to λ = 2 by a prescribed displacement in a nonlinear static
+step. The first iterations of an increment strain only the elements next to the moving face,
+whose mixed tangents are then indefinite: the step relies on the LDLᵀ factorization (before it,
+every increment was cut back until the step hit its cap). The reference is the compressible
+decoupled energy's uniaxial stress with the lateral stretch found by bisection so σ_yy = 0.

tests/oracle/nl-hybrid-incompressible/expected.json

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+{
+ "title": "Nearly incompressible hybrid block under prescribed stretch",
+ "kind": "theory",
+ "source": "Uniaxial stress of the decoupled compressible neo-Hookean energy W = μ/2 (Ī₁ − 3) + κ/2 (J − 1)² (P. J. Flory, Trans. Faraday Soc. 57:829–838, 1961): σ = μ J^(−5/3) (λ² − I₁/3) + κ (J − 1) with the lateral stretch found so σ_yy = 0 (bisection; E = 3, ν = 0.4999: μ = 1.0000667, κ = 5000)",
+ "quantities": [
+  {
+   "id": "sigma",
+   "quantity": "axial Cauchy stress, stretch λ = 2, ν = 0.4999",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Uniaxial",
+     "at": [
+      [
+       1,
+       1,
+       1
+      ]
+     ]
+    },
+    "pointer": "/nodes/0/stress/0"
+   },
+   "value": 3.498756466061613,
+   "tolerance": {
+    "relative": 1e-06
+   },
+   "reason": "a homogeneous deformation every hybrid hex8 represents exactly; the 27-element mesh makes the global mixed tangent indefinite in the first iterations (LDLᵀ): the tolerance admits the Newton tolerance and rounding"
+  },
+  {
+   "id": "lateral",
+   "quantity": "lateral displacement at (1, 1, 1): l − 1",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Uniaxial",
+     "at": [
+      [
+       1,
+       1,
+       1
+      ]
+     ]
+    },
+    "pointer": "/nodes/0/displacement/1"
+   },
+   "value": -0.29281075714096483,
+   "tolerance": {
+    "relative": 1e-06
+   },
+   "reason": "a homogeneous deformation every hybrid hex8 represents exactly; the 27-element mesh makes the global mixed tangent indefinite in the first iterations (LDLᵀ): the tolerance admits the Newton tolerance and rounding"
+  }
+ ]
+}

tests/oracle/nl-hybrid-incompressible/fixture.json

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+{
+  "note": "FieldCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "lateral": {
+      "band": 2.9281075714096485e-10,
+      "value": -0.29281075714062965
+    },
+    "sigma": {
+      "band": 3.4987564707960495e-9,
+      "value": 3.4987564707960495
+    }
+  }
+}

tests/oracle/nl-hybrid-incompressible/recipe.json

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+[
+ {"command": "mesh.block", "params": {"body": "Cube", "kind": "hex8", "corners": [[0, 0, 0], [1, 0, 0], [1, 1, 0], [0, 1, 0], [0, 0, 1], [1, 0, 1], [1, 1, 1], [0, 1, 1]], "divisions": [3, 3, 3], "faces": {"6": "x0", "3": "y0", "1": "z0", "4": "x1"}}},
+ {"command": "model.set.create", "params": {"name": "all", "geometry": [{"body": "Cube"}]}},
+ {"command": "model.set.create", "params": {"name": "x0", "geometry": [{"body": "Cube", "face": "x0"}]}},
+ {"command": "model.set.create", "params": {"name": "y0", "geometry": [{"body": "Cube", "face": "y0"}]}},
+ {"command": "model.set.create", "params": {"name": "z0", "geometry": [{"body": "Cube", "face": "z0"}]}},
+ {"command": "model.set.create", "params": {"name": "x1", "geometry": [{"body": "Cube", "face": "x1"}]}},
+ {"command": "model.material.create", "params": {"name": "Rubber", "elastic": {"type": "isotropic", "young": 3.0, "poisson": 0.4999}, "hyperelastic": {"type": "neo_hookean"}}},
+ {"command": "model.section.create", "params": {"name": "cube", "set": "all", "material": "Rubber", "kind": {"type": "solid"}, "formulation": {"hybrid": true}}},
+ {"command": "model.boundary_condition.create", "params": {"name": "sx", "type": "symmetry", "target": {"set": "x0"}, "plane": "x"}},
+ {"command": "model.boundary_condition.create", "params": {"name": "sy", "type": "symmetry", "target": {"set": "y0"}, "plane": "y"}},
+ {"command": "model.boundary_condition.create", "params": {"name": "sz", "type": "symmetry", "target": {"set": "z0"}, "plane": "z"}},
+ {"command": "model.boundary_condition.create", "params": {"name": "pull", "type": "displacement", "target": {"set": "x1"}, "values": [1.0, null, null, null, null, null]}},
+ {"command": "model.step.create", "params": {"name": "Uniaxial", "procedure": {"type": "nonlinear_static"}, "boundary_conditions": ["sx", "sy", "sz", "pull"], "loads": []}},
+ {"command": "solve.nonlinear", "params": {"step": "Uniaxial"}, "id": "solve"}
+]