Unit: digest selection with known vectors (xxh64 seed 0/empty, md5 RFC vectors, seed changes xxh64 but not md5, name mapping, bounded buffer rejection), CLI parse (all spellings, = forms, --cc alias, unsupported rejected, seed strict decimal), config wire round-trip and out-of-range algo rejection, and seeded delta signature/compute symmetry (matching seed rebuilds, mismatched seed yields no block matches). Integration: unchanged skip + same-size/mtime redetect for xxh64 and md5 with and without -m, near-zero-data skip run, deterministic seed, and a byte-exact seeded delta run.