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/client/client.c

@@ -17,9 +17,18 @@
 #include <dirent.h>
 
 int send_chunk(Client *client, Chunk *chunk, Config *config) {
-  if (config->use_compression && config->use_chunk_serialization) {
-    Data *data = chunk_compress(chunk, config->compression_level);
+  if (config->use_chunk_serialization) {
+    send_status(client->file_descriptor, STATUS_CHUNK);
+    Data *data;
+    if (config->use_compression) {
+      data = chunk_compress(chunk, config->compression_level);
+    } else {
+      for (int i = 0; i < chunk->element_count; i++)
+        file_load_data(chunk->items[i]);
+      data = chunk_serialize(chunk);
+    }
     send_data(client->file_descriptor, data->data, data->size);
+    data_destroy(data);
   } else {
     for (int i = 0; i < chunk->element_count; i++) {
       send_status(client->file_descriptor, STATUS_NEXT);