Conductivity

Thermal conductivity at the interface between two adjacent thermal layers of the urban canopy column. Implements the conductivity-weighted harmonic mean (Oleson et al. 2010, CLM Tech Note Ch. 4, Eq. 4.12) derived from continuity of conductive heat flux across the interface (Eq. 4.13). Migrated from UrbanCanopy.jl@9200edd4 src/roof_wall_road_snow_temperatures.jl:278 (`InterfaceThermalConductivity` @component). Parameterized as observed-only: λ_i, λ_ip1, z_i, z_ip1, z_h are inputs (parameters); λ_interface is the algebraic output.