IV.T160 — θ_QCD = 0 from C-Sector SA-i Admissibility

θ_QCD = 0 exactly from SA-i. Proof: CP violation ↔ Q_top ≠ 0 ↔ Δ(η-winding) ≢ 0 mod 3 ↔ SA-i violated. All three steps are structural consequences of the τ-axioms. Lean skeleton: sa_i_forbids_instantons proved by decide (1%3≠0 ∧ (-1)%3≠0).