Test B: sensitivity of the ratio-τ relation to coherent scaling of all isotopic ratios by ±30% around the ISM values. Top panels: absolute C¹⁷O/¹³C¹⁸O(2–1) (left) and C¹⁸O/C¹⁷O(1–0) (right) ratios as a function of line-center optical depth. Bottom panels: normalized curves collapse to within ≲1%–2% for τ ≲ 0.3, confirming the robustness of our optical depth diagnostic and thin-region identification.