IV.P210 — First-Row CKM Unitarity (Cabibbo Anomaly Test)

|V_ud|²+|V_us|²+|V_ub|² = 1+λ⁴/4+O(λ⁶) ≈ 1.000651. Unitarity exact by construction (Wolfenstein). The +651 ppm excess is O(λ⁴) truncation artifact. Experimental Cabibbo anomaly (3σ deficit, 0.9986±0.0005) would require NLO correction to λ=ι_τ(1−ι_τ) if confirmed. Current evidence: nuclear structure corrections, not new physics.