feat: implement chunk serialization protocol with -s flag

- Extract chunk_serialize/chunk_deserialize from chunk_compress/chunk_decompress
- Add STATUS_CHUNK to wire protocol for chunk-mode transfers
- Client: -s sends chunk via STATUS_CHUNK (serialized or compressed+serialized)
- Server: handle STATUS_CHUNK in both single-threaded and multithreaded paths
- Fix memory leak in receive_file_receive (free->data_destroy)
- Remove dead declarations: file_receive_from_buffer, file_receive_decompress,
  file_compress, chunk_data_to_disk, unused #defines

src/shared/socket.c

@@ -195,6 +195,8 @@ const char *status_to_string(Status status) {
     return "FINISHED";
   case STATUS_NEXT:
     return "NEXT";
+  case STATUS_CHUNK:
+    return "CHUNK";
   default:
     return "UNKNOWN";
   }