Viscous conductive heat flux

Face energy residual includes a conductive contribution aligned with the temperature normal gradient.

Effective conductivity

\[k = \Big(\mu_l / Pr_l + \mu_t / Pr_t\Big) c_p\]

\[q_n = \mathbf{q}\cdot\mathbf{n} + k\,\partial_n T\]

With Cartesian heat-vector components currently zero in the laminar path, the dominant term is \(k\partial_n T\).

Pure APIs: EffectiveThermalConductivity, ConductiveHeatFluxNormal, FaceNormalDot (NsVisFluxPolicy.h). Tests: vis_flux_policy_test.