refactor: split monolithic modules and extract shared components

- Remove dead socket.c/socket.h (accumulated duplicate symbols)
- Extract compression.h/c (data_compress/decompress from data.c)
- Extract io.h/c (low-level I/O from protocol.c)
- Extract metadata.h/c (unified metadata wire format from file.c, chunk.c, utils.c)
- Split client.c into client_cli.c (CLI parsing) + client_send.c (send logic)
- Move receiver pipeline threads from server.c to multiprocessing.c
- Move to_disk/file_restore_metadata from utils.c to file.c/metadata.c
- Fix const qualifiers on to_disk and str_dup signatures
- Suppress chown unused-result warning

src/shared/data.h

@@ -12,7 +12,5 @@ Data *data_create_empty(size_t data_size);
 Data *data_create_reserve(size_t size);
 Data *data_create(void *data, size_t data_size);
 void data_destroy(Data *data);
-Data *data_compress(Data *data_to_compress, int compression_level);
-Data *data_decompress(Data *compressed_data);
 
 #endif