IV.P225 — c₂ Numerical Bound from W₄(3)

Two-loop EW correction: sin²θ_W(NNLO) × (1+ι_τ²/18) gives ~6470 ppm shift. For α, two-loop is α·c₂·ι_τ² ≈ 0.5 ppm (sub-1 ppm). Two-loop corrections negligible for α but relevant for EW mixing.