Hatta_number

Correction factor Q for the aqueous-phase reaction rate when intra-droplet diffusion limits access to the reactive interior (Seinfeld & Pandis 2006, Chapter 12, Eqs. 12.105 and 12.110). Computes the dimensionless Hatta number q = R_p √(k_rxn/D_aq) and the corresponding correction factor Q = 3(q·coth q − 1)/q², which scales the effective reaction rate from the well-mixed limit (Q = 1) down toward zero as the droplet becomes increasingly diffusion-limited. Migrated from Aerosol.jl src/aqueous_transport.jl:14.