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/protocol.h

@@ -0,0 +1,20 @@
+#ifndef PROTOCOL_H
+#define PROTOCOL_H
+
+#include "data.h"
+#include "io.h"
+#include <stddef.h>
+
+typedef int Status;
+enum NET_STATUS { STATUS_OK, STATUS_ERROR, STATUS_FINISHED, STATUS_NEXT, STATUS_CHUNK };
+
+void send_str(int file_descriptor, char *data);
+char *receive_str(int file_descriptor);
+void send_data(int file_descriptor, void *data, unsigned long long data_size);
+Data *receive_data(int file_descriptor);
+void send_int(int file_descriptor, int data);
+int receive_int(int file_descriptor);
+void send_status(int file_descriptor, Status status);
+Status receive_status(int file_descriptor);
+
+#endif