Tolerances (HARNESS.md): rel_L∞ ≤ 1e-3 (blowup detector, 1 mm floor), rel_L2(wet) ≤ 1e-4 (the tight physics gate), extent Jaccard ≤ 0.02. Reference: desktop GPU production path @ 36adb05. The desktop is canonical — a failure here means the browser port is wrong.