V.T225 — Echo Detection Threshold

SNR bound: SNR_echo ≤ 0.036 × SNR_main (τ-effective). Approx: SNR_echo ~ 0.043 × SNR_main (conjectural). GW150914: ~1.04. Stacking ≥19 events: SNR ≥ 3. ET: single-event ~10.4.