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/chunk.c

@@ -1,17 +1,15 @@
-#include <dirent.h>
-#include <libgen.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-#include "chunk.h"
 #include "array_list.h"
+#include "chunk.h"
+#include "compression.h"
 #include "data.h"
 #include "file.h"
 #include "log.h"
-
-#define FILE_METADATA_WIRE_SIZE (sizeof(mode_t) + sizeof(uid_t) + sizeof(gid_t) + sizeof(time_t) + sizeof(long))
+#include "metadata.h"
 
 Chunk *chunk_create(File **items, int element_count) {
   Chunk *chunk = (Chunk *)malloc(sizeof(Chunk));
@@ -49,34 +47,6 @@ void chunk_destroy(void *item) {
   free(chunk);
 }
 
-static 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);
-}
-
-static 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;
-}
-
 static unsigned long long per_file_serialize_size(File *file, bool use_metadata) {
   return sizeof(size_t) + strlen(file->path) +
          (use_metadata ? sizeof(int) + (file->metadata ? FILE_METADATA_WIRE_SIZE : 0) : 0) +