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.c

@@ -127,6 +127,10 @@ static const char *status_to_string(Status status) {
     return "CHUNK";
   case STATUS_CHECK:
     return "CHECK";
+  case STATUS_DELTA_SIGNATURE:
+    return "DELTA_SIGNATURE";
+  case STATUS_DELTA_DATA:
+    return "DELTA_DATA";
   default:
     return "UNKNOWN";
   }