IV.T206 — θ₂₃ NNLO from Holonomy-at-Window-Squared

sin(θ₂₃) = (1−ι_τ⁵)/(1+ι_τ)·(1−ι_τ²/W₃(4)²) = (1−ι_τ⁵)/(1+ι_τ)·(1−ι_τ²/25). sin²θ₂₃ = 0.5457 at −494 ppm from PDG 0.546. 94% improvement from NLO +8604. Universal NNLO pattern: W₃(4)²=25 shared with m_μ/m_e.