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.h

@@ -5,7 +5,7 @@
 #include <netinet/in.h>
 
 typedef int Status;
-enum NET_STATUS { STATUS_OK, STATUS_ERROR, STATUS_FINISHED, STATUS_NEXT };
+enum NET_STATUS { STATUS_OK, STATUS_ERROR, STATUS_FINISHED, STATUS_NEXT, STATUS_CHUNK };
 
 typedef struct Server {
   struct sockaddr_in address;