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/transport_tcp.h

@@ -0,0 +1,28 @@
+#ifndef TRANSPORT_TCP_H
+#define TRANSPORT_TCP_H
+
+#include <netinet/in.h>
+#include <sys/types.h>
+
+typedef struct Server {
+  struct sockaddr_in address;
+  unsigned int address_length;
+  int file_descriptor;
+} Server;
+
+typedef struct Client {
+  struct sockaddr_in address;
+  unsigned int address_length;
+  int file_descriptor;
+  pid_t ssh_child_pid;
+} Client;
+
+Server *server_create(int port);
+void server_listen(Server *server, void (*handler)(int file_descriptor));
+void server_delete(Server **server);
+Client *client_create();
+void client_connect(Client *client, char *host, int port);
+void client_disconnect(Client *client);
+void client_delete(Client *client);
+
+#endif