Precipitation_phase

Offline-mode atmospheric forcing processor for the Community Land Model Urban (CLMU). Migrated from UrbanCanopy.jl@9200edd4 src/offline_mode.jl:33 (`OfflineModeForcing` @component). Implements Oleson et al. (2010, NCAR Tech Note CLMU Ch. 6): given the raw observational inputs total incident solar radiation S_atm, atmospheric temperature T_atm, pressure P_atm, specific humidity q_atm, wind speed W_atm, and total precipitation rate P_precip, it derives (a) the four-way solar partition into direct/diffuse × visible/NIR via the Eq. 6.7-6.8 cubic polynomials and Eqs. 6.2-6.5 visibility / NIR weighting; (b) the downwelling longwave radiation L_atm_down from the Idso (1981) formula (Eqs. 6.9-6.10) including the vapor-pressure conversion from specific humidity; (c) the rain/snow phase partition from a linear T_atm - T_f ramp (Eqs. 6.11-6.13); and (d) the wind components, potential temperature, and atmospheric reference height (p.149) needed by CLMU's surface-layer routines. Eq. 6.1 (temporal downscaling of 6-hourly solar to model time steps) and Eqs. 6.14-6.15 (specific humidity from RH / dewpoint) are upstream data-preprocessing steps and are intentionally not implemented here; S_atm and q_atm are taken as already at the model time step. Parameterized as observed-only with 18 algebraic equations and 18 state-variable outputs — no time derivatives, no `@register_symbolic` dependencies, no PDE — matching the upstream `length(equations(sys)) == 18` / `length(unknowns(sys)) == 18` structural tests.