Nonlinear Voigt rule of mixtures as a weighted_sum wiring

[petlenz]

Part of #117 (the Voigt half) and of the uvwmaterial port (#94); stacked on #139.

small_strain_rule_of_mixture_history with the Voigt kernel (equal strain in every phase, history per phase) needs no new material: each phase reads the composite strain source, and weighted_sum with constant_scalar fractions produces Σ cᵢ σᵢ and Σ cᵢ Cᵢ — the consistent Voigt tangent. This PR pins that with tests and documents it in the mapping.

Tests (test_voigt_composite, 2): a J2 phase and an elastic phase under uniaxial stress through the driver — the composite stress equals the fraction-weighted phase stresses at every step while the plastic phase yields, and the tangent matches central differences to 1e-6; while elastic the wiring equals the linear rule of mixtures (phase nodes on the two tangents, rule_of_mixture voigt, linear_stress) in stress and tangent.

Not in this PR: the Reuss kernel (equal stress, phase strains from an inner Newton with history) waits for the vector Newton work in flight; and weighted_sum cannot be written in JSON yet because the parameter converter has no reader for its terms (vector of string pairs), so there is no report for this wiring — both noted on #117. The converter (#139) emits the same Voigt wiring as rule_of_mixture + mean_field_stress for a card with a nonlinear phase.

942/942 tests pass (GCC 14, reports on).

[petlenz]

Review findings (design, tmech expressions instead of component loops, modern C++, comments, tests, correctness). Severity: must = wrong result, hang, build break or unreachable feature; should = design, duplication, accuracy; nit = style.

Voigt composite wiring — ready (with one should)

• [should] json_parameter_converter.h:117 already has variant<double, pair<string,string>>; a vector<pair<string,string>> reader is small and would let this ship docs/materials/voigt_composite/report.json instead of "not from JSON yet".

[petlenz]

Rebased: the linear reference is a rule_of_mixture over two inclusion phases fed by the same c_plastic/c_elastic constants the weighted_sum wiring uses.