Remove 7490-line vendored xxhash.h, use FetchContent to pull xxHash v0.8.3
at CMake configure time (SOURCE_SUBDIR cmake_unofficial). Links as static
library via the xxhash target.
- Fix rolling adler32: add missing -1 in s2 update formula (was producing
wrong checksums, causing zero block matches)
- Fix file_send_sendfile signature to match typedef (add unused
compression_level param)
- Extract receive_delta_file() from receive_incremental_check() to reduce
nesting depth
- Extract send_file_incremental() helper in client_send.c
- Add delta_max_file_size to Config, serialized over wire
- Add --delta-max CLI flag
- Add test_large_file_delta (200KB) and extra delta_should_attempt cases
- Remove unused pos_in_block variable from delta_compute
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
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