Heat_flux

Vertical heat flux across the interface between two soil/snow/road/wall/roof layers via the finite-difference discretization of Fourier's law of heat conduction (Oleson et al. 2010 Eq. 4.11). Migrated from UrbanCanopy.jl@9200edd4 src/roof_wall_road_snow_temperatures.jl:312 (`HeatFlux` @component). Implements the canonical layer-pair flux F = -λ_interface * (T_i - T_{i+1}) / (z_{i+1} - z_i), positive upward (heat flowing from layer i+1 to layer i, i.e. from deeper layer to shallower layer when stacking depth-increasing-downward). λ_interface is the harmonic-mean interface conductivity (Eq. 4.12), produced upstream by InterfaceThermalConductivity from the adjacent-layer bulk conductivities. Parameterized as observed-only with one algebraic equation: F is the state variable; λ_interface, T_i, T_ip1, z_i, z_ip1 are inputs (parameters in standalone form, wired to upstream components in a full column build).