Add real implementations for the rsync 3.4.1 checksum and compression
breadth: a self-contained MD4 (RFC 1320), OpenSSL-backed SHA1, a
no-digest mode, and LZ4/zlib codecs alongside zstd. Compressed buffers
are now self-describing (a leading codec id), so every existing
decompression call site keeps working through a process-global codec
selection. zlibx shares the zlib codec because FastSync compresses only
delta/token bytes (never matched file data), matching the 'x' intent.