Certified critical-line bound 0.673591770245738332299780842385

Uses Samuel Lavery's attributed attempt-030 W9A2 certificate, with the same nine-point window, weights, certificate data, c, B, and rational bounds for pi, sqrt(2)/2, sine and cosine. Only the terminal H_W floor is sharpened from 67212374687824171413662471723172/10^32 to 672123746878241714136624717231727/10^33. The sharper rational is propagated through the existing actual dyadic and cumulative zero-counting proofs to the stated exact p/q. All original attribution and license notices are retained; the file adds a notice identifying Li Xiang's change.