V.T271 — 21cm Absorption Trough from tau-Native Inputs

At z≈17 with complete Wouthuysen–Field coupling and full neutrality: T₂₁(z=17)≈−209 mK. Standard ΛCDM prediction evaluated with τ-native parameters. Adiabatic gas T_K(17)≈6.8 K vs T_CMB(17)≈49 K. Testable by HERA and SKA.