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/compression.h

@@ -0,0 +1,9 @@
+#ifndef COMPRESSION_H
+#define COMPRESSION_H
+
+#include "data.h"
+
+Data *data_compress(Data *data_to_compress, int compression_level);
+Data *data_decompress(Data *compressed_data);
+
+#endif