λ_P = 4σT_e³ (exact symbolic derivative)

Closed-form symbolic derivative of the Stefan-Boltzmann law. OLR(T_e) = σT_e⁴; dOLR/dT_e = 4σT_e³ — a cubic monomial. Sympy symbolic differentiation yields the canonical string "4*T_e**3*sigma" exactly; no numerical finite-difference…