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/config.h

@@ -2,6 +2,7 @@
 #define CONFIG_H
 
 #include <stdbool.h>
+#include <stdint.h>
 
 typedef enum {
   TRANSPORT_TCP,
@@ -33,13 +34,15 @@ typedef struct Config {
   unsigned long long max_size;
   unsigned long long min_size;
   bool use_incremental;
+  bool use_delta;
+  uint32_t delta_block_size;
   bool use_tls;
   char *tls_cert;
   char *tls_key;
   char *tls_ca;
 } Config;
 
-#define PROTOCOL_VERSION "1.1.0"
+#define PROTOCOL_VERSION "1.2.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 
 Config *config_create(char *version, char *send_directory,