Stiff-Ode-Benchmark

POLLU air-pollution photochemistry mechanism from GasChem.jl. 20 species and 25 reactions (15 thermal + 10 photolytic) describing tropospheric NOx / odd-oxygen / hydrocarbon / sulfur chemistry. Introduced by Verwer (1994) as a stiff-ODE benchmark for chemical-kinetics integrators and widely used since as a standard stiff test problem. Hand-translated from the upstream Catalyst @reaction_network in GasChem.jl src/Pollu.jl: 25 rate constants carried verbatim as parameters, 20 species with their declared reservoir defaults, and 25 mass-action reactions numbered R1..R25 in the canonical POLLU order. All reaction rates are bare parameter references and all substrate stoichiometries are 1, so the serialized mass-action system matches the upstream combinatoric_ratelaws=false ReactionSystem exactly.