IV.R421 — Light Quark Mass Ratio m_u/m_d Candidates

m_u/m_d≈0.46±0.03 (lattice). Best candidates: ι_τ^0.72=0.465 (tautological), κ_D·ι_τ^{-1/3}=0.463 (no derivation), 1/2−2ι_τ/17=0.460 (+535 ppm, not convincing). No clean sub-1% formula found. Requires confinement-sector structure.