mean_field_strain and mean_field_stress: strain localisation around the phase materials (linear)

[petlenz]

Part of #116 (the linear half). Part of the uvwmaterial port (#94); stacked on #141 (the whole stack: Eshelby and depolarisation nodes, phase nodes, producer/consumer wiring).

Strain localisation as two nodes around the phase materials. Nothing walks the graph, nothing re-binds relays; the engine orders everything and the materials are ordinary consumers of a produced strain.

• mean_field_strain: from the composite strain increment (strain_source, history) and the phases' tangents, fractions and Eshelby tensors (matrix, phases: phase nodes) it produces one strain per phase, strain_<phase> = ε_r,ₙ + A_r : Δε (history properties: commit, revert and the UMAT state vector carry them), and the concentrations concentration_<phase>. kernel is any of the global schemes — voigt (A = I, no tangents read), reuss, hill (Aᵣ = ℂᵣ⁻¹ : ℂ_R and the mean with I, no Eshelby tensors), dilute, mori_tanaka — so the rule of mixtures localises through the same pair.
• The phase materials consume those strains like any other: "strain_source": "mt_strain", "strain_property": "strain_matrix" (materials: strain consumers name the strain property they read #145). Here linear_stress / linear_elasticity on the producers' tangents — evaluated once per update, at their local strain.
• mean_field_stress: stress = Σ c_r σ_r and tangent = Σ c_r C_r : A_r from the materials' current stresses and tangents (matrix_stress_source, stress_sources, optional tangent_sources, matrix_stress_property / matrix_tangent_property for a material that names them differently) and the concentrations of the strain node; residual = the largest relative difference between a phase node's tangent and its material's current one — identically zero here, the handle the nonlinear PR uses.

With linear phases every edge is global, one pass is the fixed point and the result equals mean_field_tangent + linear_stress to round-off, with the phase strains and stresses available as properties. The nonlinear case (a phase whose tangent depends on its own strain) is the next PR: a local tangent edge on the phase node and the passes closing the fixed point.

Tests (test_mean_field_local, 4): the linear wiring converges in one pass (residual = 0) and equals the tangent homogenisation (tangent, stress, matrix strain); Voigt, Reuss and Hill through the pair equal rule_of_mixture (tangent, concentrations, stress; Voigt: equal strain, Reuss: equal stress); non-parallel source lists are rejected; JSON.

Docs: docs/mean_field.md (the composite layer: phase nodes, tangent homogenisation, the self-consistent cycle, strain localisation, who runs the fixed point) and the README links.

965/965 tests pass (GCC 14, reports on).

JSON input

{"materials": [
  {"type": "external_strain_source", "name": "strain"},
  {"type": "constant_scalar", "name": "Km", "value": 3.0},  {"type": "constant_scalar", "name": "Gm", "value": 1.0},
  {"type": "constant_scalar", "name": "Ki", "value": 60.0}, {"type": "constant_scalar", "name": "Gi", "value": 30.0},
  {"type": "isotropic_tangent", "name": "C_m", "K_source": "Km", "G_source": "Gm"},
  {"type": "isotropic_tangent", "name": "C_i", "K_source": "Ki", "G_source": "Gi"},
  {"type": "isotropic_moduli", "name": "moduli", "tangent_source": "C_m"},
  {"type": "eshelby_sphere", "name": "sphere", "nu_source": "moduli", "nu_property": "poisson_ratio"},
  {"type": "constant_scalar", "name": "c", "value": 0.3},
  {"type": "constant_scalar", "name": "cm", "value": 0.7},
  {"type": "phase", "name": "matrix", "tangent_source": "C_m", "fraction_source": "cm"},
  {"type": "phase", "name": "spheres", "tangent_source": "C_i", "fraction_source": "c", "eshelby_source": "sphere"},
  {"type": "mean_field_strain", "name": "mt_strain", "strain_source": "strain", "matrix": "matrix", "phases": ["spheres"]},
  {"type": "linear_stress", "name": "m", "tangent_source": "C_m", "strain_source": "mt_strain", "strain_property": "strain_matrix"},
  {"type": "linear_stress", "name": "s", "tangent_source": "C_i", "strain_source": "mt_strain", "strain_property": "strain_spheres"},
  {"type": "mean_field_stress", "name": "mt", "concentration_source": "mt_strain", "matrix": "matrix", "phases": ["spheres"],
   "matrix_stress_source": "m", "matrix_tangent_source": "C_m", "stress_sources": ["s"], "tangent_sources": ["C_i"]}
]}