IV.T156 — m_μ/m_e NNLO Correction: δ = 1/W₃(4)² = 1/25

The muon-to-electron mass ratio correction exponent δ = 1/25 = 1/W₃(4)² = 0.04 exactly. Exponent = −5 + 1/25 = −124/25. Formula ι_τ^(−124/25) = 206.832 at +307 ppm from PDG 206.768. NNLO Window Rule: W₃(4)=5 (NLO) → W₃(4)²=25 (NNLO). Cross-check: 4×W₃(4)+1 = 21 appears in p-n mass difference bonus coefficient 1/21.