feat: add delta transfer for incremental sync

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

src/shared/protocol.h

@@ -8,7 +8,7 @@
 typedef struct ssl_st SSL;
 
 typedef int Status;
-enum NET_STATUS { STATUS_OK, STATUS_ERROR, STATUS_FINISHED, STATUS_NEXT, STATUS_CHUNK, STATUS_MANIFEST, STATUS_CHECK };
+enum NET_STATUS { STATUS_OK, STATUS_ERROR, STATUS_FINISHED, STATUS_NEXT, STATUS_CHUNK, STATUS_MANIFEST, STATUS_CHECK, STATUS_DELTA_SIGNATURE, STATUS_DELTA_DATA };
 
 void io_set_fds(int read_fd, int write_fd);
 void io_set_bwlimit(unsigned long long bytes_per_sec);