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

tests/runner.c

@@ -2,6 +2,7 @@
 #include "test_chunk.h"
 #include "test_compression.h"
 #include "test_config.h"
+#include "test_delta.h"
 #include "test_queue.h"
 #include "test_scanner.h"
 #include "test_shared_utils.h"
@@ -23,6 +24,7 @@ int main() {
   RUN_TEST(test_config);
   RUN_TEST(test_compression);
   RUN_TEST(test_scanner);
+  RUN_TEST(test_delta);
 
   printf("\n\033[1;36m=== TEST SUMMARY ===\033[0m\n");
   printf("Total Tests Run: %d\n", tests_run);