IV.R405 — Comparison to Peccei-Quinn: SA-i as τ-Native PQ Mechanism

Comparison table: PQ (axion, f_PQ free, m_a~10⁻⁵ eV), Nelson-Barr (mediators), τ/SA-i (no new fields, f_PQ fixed = κ(C;3)·m_n). SA-i is the only solution derived from first principles without additional parameters. The τ prediction (no axion) is falsifiable via ADMX/CASPEr.