V.T188 — η_B Formal Derivation: α·ι_τ¹⁵·(5/6) at −10320 ppm

Full formal derivation of η_B = α·ι_τ^15·(5/6): α = EM coupling at threshold, ι_τ^15 = SA-i mod-5 suppression from 5-fold holonomy winding (V.D245), (5/6) = W₃(4)/(2·|sectors|) generator/sector ratio (V.T180). Result: η_B(τ) = 6.041×10⁻¹⁰ vs Planck CMB-only (6.104±0.058)×10⁻¹⁰, deviation −10320 ppm (within 1.1σ CMB-only). Upgrades Sprint 6C from 'proposed' to 'formally derived'.