Probe shows the block-checksum choice is not observable in the parity surface:
%c, Matched/Literal data and the destination tree are invariant across
xxh64/xxh128/xxh3/md5/md4/sha1 and the transfer,pre-transfer form; only %C
changes, and it is byte-identical to rsync. FastSync's fixed xxHash32 block
strong sum is collision-safe within the payload cap. Row -> parity (114/11/32).
Add tests/integration/test_codecs.py (accept/reject matrix and byte
differential against rsync 3.4.1 for every algorithm), extend the
checksum/compression unit tests with MD4/SHA1/none vectors and per-codec
round-trips, pin the new config golden (2.26.0), and update the version
strings and codec acceptance expectations.