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.c

@@ -0,0 +1,79 @@
+#include "protocol.h"
+#include "io.h"
+#include "log.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static const char *status_to_string(Status status) {
+  switch (status) {
+  case STATUS_OK:
+    return "OK";
+  case STATUS_ERROR:
+    return "ERROR";
+  case STATUS_FINISHED:
+    return "FINISHED";
+  case STATUS_NEXT:
+    return "NEXT";
+  case STATUS_CHUNK:
+    return "CHUNK";
+  default:
+    return "UNKNOWN";
+  }
+}
+
+void send_str(int file_descriptor, char *data) {
+  size_t size = strlen(data);
+  send_n_data(file_descriptor, &size, sizeof(size_t));
+  send_n_data(file_descriptor, data, size);
+  log_message(LOG_LEVEL_DEBUG, "Send String: %s", data);
+}
+
+char *receive_str(int file_descriptor) {
+  size_t size;
+  receive_n_data(file_descriptor, &size, sizeof(size_t));
+  char *data = (char *)malloc(size + 1);
+  receive_n_data(file_descriptor, data, size);
+  data[size] = '\0';
+  log_message(LOG_LEVEL_DEBUG, "Received String: %s", data);
+  return data;
+}
+
+void send_data(int file_descriptor, void *data, unsigned long long data_size) {
+  send_n_data(file_descriptor, &data_size, sizeof(unsigned long long));
+  send_n_data(file_descriptor, data, data_size);
+  log_message(LOG_LEVEL_DEBUG, "Send %lld data", data_size);
+}
+
+Data *receive_data(int file_descriptor) {
+  size_t size = 0;
+  receive_n_data(file_descriptor, &size, sizeof(unsigned long long));
+  void *data = malloc(size);
+  receive_n_data(file_descriptor, data, size);
+  log_message(LOG_LEVEL_DEBUG, "Received %lld data", size);
+  return data_create(data, size);
+}
+
+void send_int(int file_descriptor, int data) {
+  send_n_data(file_descriptor, &data, sizeof(int));
+  log_message(LOG_LEVEL_DEBUG, "Send Int: %d", data);
+}
+
+int receive_int(int file_descriptor) {
+  int data;
+  receive_n_data(file_descriptor, &data, sizeof(int));
+  log_message(LOG_LEVEL_DEBUG, "Received Int: %d", data);
+  return data;
+}
+
+void send_status(int file_descriptor, Status status) {
+  send_n_data(file_descriptor, &status, sizeof(Status));
+  log_message(LOG_LEVEL_DEBUG, "Send Status: %s", status_to_string(status));
+}
+
+Status receive_status(int file_descriptor) {
+  Status data;
+  receive_n_data(file_descriptor, &data, sizeof(Status));
+  log_message(LOG_LEVEL_DEBUG, "Received Status: %s", status_to_string(data));
+  return data;
+}