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/metadata.c

@@ -0,0 +1,82 @@
+#include "metadata.h"
+#include "file.h"
+#include "io.h"
+#include <fcntl.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <time.h>
+#include <unistd.h>
+
+#define FILE_METADATA_WIRE_SIZE (sizeof(mode_t) + sizeof(uid_t) + sizeof(gid_t) + sizeof(time_t) + sizeof(long))
+
+void metadata_to_buf(char **buf, FileMetadata *m) {
+  int present = (m != NULL) ? 1 : 0;
+  memcpy(*buf, &present, sizeof(int));
+  *buf += sizeof(int);
+  if (m == NULL)
+    return;
+  memcpy(*buf, &m->mode, sizeof(mode_t));        *buf += sizeof(mode_t);
+  memcpy(*buf, &m->uid, sizeof(uid_t));          *buf += sizeof(uid_t);
+  memcpy(*buf, &m->gid, sizeof(gid_t));          *buf += sizeof(gid_t);
+  memcpy(*buf, &m->mtime_sec, sizeof(time_t));   *buf += sizeof(time_t);
+  memcpy(*buf, &m->mtime_nsec, sizeof(long));    *buf += sizeof(long);
+}
+
+FileMetadata *metadata_from_buf(char **buf) {
+  int present;
+  memcpy(&present, *buf, sizeof(int));
+  *buf += sizeof(int);
+  if (!present)
+    return NULL;
+  FileMetadata *m = malloc(sizeof(FileMetadata));
+  memcpy(&m->mode, *buf, sizeof(mode_t));        *buf += sizeof(mode_t);
+  memcpy(&m->uid, *buf, sizeof(uid_t));          *buf += sizeof(uid_t);
+  memcpy(&m->gid, *buf, sizeof(gid_t));          *buf += sizeof(gid_t);
+  memcpy(&m->mtime_sec, *buf, sizeof(time_t));   *buf += sizeof(time_t);
+  memcpy(&m->mtime_nsec, *buf, sizeof(long));    *buf += sizeof(long);
+  return m;
+}
+
+void metadata_send(int file_descriptor, FileMetadata *m) {
+  if (m == NULL) {
+    int zero = 0;
+    send_n_data(file_descriptor, &zero, sizeof(int));
+    return;
+  }
+  int present = 1;
+  send_n_data(file_descriptor, &present, sizeof(int));
+  send_n_data(file_descriptor, &m->mode, sizeof(mode_t));
+  send_n_data(file_descriptor, &m->uid, sizeof(uid_t));
+  send_n_data(file_descriptor, &m->gid, sizeof(gid_t));
+  send_n_data(file_descriptor, &m->mtime_sec, sizeof(time_t));
+  send_n_data(file_descriptor, &m->mtime_nsec, sizeof(long));
+}
+
+FileMetadata *metadata_receive(int file_descriptor) {
+  int present;
+  receive_n_data(file_descriptor, &present, sizeof(int));
+  if (!present)
+    return NULL;
+  FileMetadata *m = malloc(sizeof(FileMetadata));
+  receive_n_data(file_descriptor, &m->mode, sizeof(mode_t));
+  receive_n_data(file_descriptor, &m->uid, sizeof(uid_t));
+  receive_n_data(file_descriptor, &m->gid, sizeof(gid_t));
+  receive_n_data(file_descriptor, &m->mtime_sec, sizeof(time_t));
+  receive_n_data(file_descriptor, &m->mtime_nsec, sizeof(long));
+  return m;
+}
+
+void file_restore_metadata(const char *path, FileMetadata *metadata) {
+  if (metadata == NULL)
+    return;
+  chmod(path, metadata->mode & 07777);
+  int chown_ret = chown(path, metadata->uid, metadata->gid);
+  (void)chown_ret;
+  struct timespec times[2];
+  times[0].tv_sec = 0;
+  times[0].tv_nsec = UTIME_OMIT;
+  times[1].tv_sec = metadata->mtime_sec;
+  times[1].tv_nsec = metadata->mtime_nsec;
+  utimensat(AT_FDCWD, path, times, 0);
+}