e5ce7e4ba3
1495 banks carry no RIFF and take a separate path that hardcoded both the offset and stereo. Across a random 48-bank sample there was NOT ONE where the old stereo-at-1392 pair beat the best mono offset; median gain 184x, individual banks going from 0-4816 decoded bytes to 180000-380000. Stereo shows the same stop-after-one-frame signature already recorded for the leading segment. With no RIFF the offset cannot be derived, so scan_data_offset picks among the four disc offsets by XMA1 packet-header plausibility. Validated on the LABELLED set -- all 7358 banks that do have a RIFF, where the answer is forced: 7330 correct (99.62%), and all 28 misses are ties on the top score, never a wrong unique winner. Ties fall back to 1392. The winning offsets also reproduce, by directory, the distribution measured independently from the RIFF-bearing banks. jpn\etc splits 1468/1600, so path alone is not sufficient -- which is why this is a scan and not a lookup table. 7 disc tests pass (build-reborn test -p sylpheed-formats --test slb_leading_segment_disc, SYLPHEED_DISC wired up).