V.D250 — CνB Temperature Chain

CνB temperature T_CνB = (4/11)^{1/3} × T_CMB = 1.9454 K from entropy conservation at e⁺e⁻ annihilation. Neutrino number density: 336 cm⁻³ total (56 per flavor per helicity). Neutrino decoupling at T_dec ≈ 1.37 MeV (z_ν ≈ 5.8×10⁹) from weak freeze-out Γ_weak = H with g_* = 10.75. The A-sector coupling κ(A;1) = ι_τ controls the weak interaction threshold.