V.T171 — (5/6) Threshold Factor Shared with Y_p = 20/81

The factor (5/6) appears in both Y_p = 20/81 = (8/27)·(5/6) (helium fraction, V.T149) and η_B = α·ι_τ¹⁵·(5/6) (baryon asymmetry). Structurally: (5/6) = 5 non-resonant threshold channels / 6 total threshold channels in the canonical ladder {L_EW, L_B, L_N, L_nuc, L_H, L_γ}. The L_B channel (baryogenesis threshold) is the unique resonant channel where the ω-crossing mediates B-violating processes.