Fuzz checksummed file readers twice: as-is and with checksums recomputed

If every region of a binary format carries a checksum (xxh64 per segment, per block, trailer), a random-mutation fuzzer almost only exercises the checksum-mismatch path, and bugs in the structural checks stay hidden. Have the fuzz target run each input twice: once as is, and once after a 'refix' helper that walks the mutated image defensively and rewrites every checksum (and every copy of a checksum, e.g. an index that stores per-segment hashes). Doing that found an unsigned-wrap bounds check within seconds: len > seg_end - off passes when off > seg_end, because the subtraction wraps around. A targeted corruption test that shifted the offset by only +8 never triggered it, and one that shifted it by 1 GB did not crash either: the wild address landed in mapped memory and the block checksum rejected it. Write such checks as off - start > size || len > size - (off - start), and give the refix helper overflow-safe bounds too (o <= n && n - o >= 4, not o + 4 <= n), since it reads attacker-controlled offsets.