Implement rsync-style delta transfer using rolling checksums (Adler-32 +
xxHash32). When a file exists on both sides but has changed, only the
changed blocks are transmitted instead of the entire file.
- New status codes: STATUS_DELTA_SIGNATURE, STATUS_DELTA_DATA
- Protocol version bumped to 1.2.0
- Server generates block signature, client computes delta
- Auto-fallback to whole-file when delta >= 70% of file size
- Works with zstd compression on delta stream
- Configurable block size (default 8KB, --delta-block flag)
- 13 unit tests covering hashing, signature roundtrip, delta compute/apply,
file growth/shrink, and decision logic
- New STATUS_CHECK protocol status (value 6)
- Client sends path + size + mtime; server replies OK (skip) or NEXT (send)
- config.h/c: use_incremental field sent/received over wire
- file.c/h: file_send_single_calls_no_path helper for incremental path
- client_cli.c: --incremental flag
- client_send.c: per-file check before send in both single and chunk paths
- server.c: STATUS_CHECK handling in receive_files()
- multiprocessing.c: STATUS_CHECK handling in receive_thread()
- test.py: incremental sync test case
- Merge io.h/c back into protocol.h/c
- Change send_data to take Data* argument
- Remove redundant file_load_data from send_chunk
- Move compression into file_send_single_calls
- Move file_receive from multiprocessing.c to file.c
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