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_ssh.c

@@ -0,0 +1,115 @@
+#include "transport_ssh.h"
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+typedef struct {
+  char user[256];
+  char host[256];
+  char remote_path[4096];
+} RemoteDest;
+
+static int parse_remote_dest(const char *dest, RemoteDest *r) {
+  const char *colon = strchr(dest, ':');
+  if (!colon) return -1;
+
+  size_t remote_path_len = strlen(colon + 1);
+  if (remote_path_len >= sizeof(r->remote_path)) return -1;
+  memcpy(r->remote_path, colon + 1, remote_path_len + 1);
+
+  const char *at = memchr(dest, '@', colon - dest);
+  if (at) {
+    size_t user_len = at - dest;
+    if (user_len >= sizeof(r->user)) return -1;
+    memcpy(r->user, dest, user_len);
+    r->user[user_len] = '\0';
+
+    size_t host_len = colon - at - 1;
+    if (host_len >= sizeof(r->host)) return -1;
+    memcpy(r->host, at + 1, host_len);
+    r->host[host_len] = '\0';
+  } else {
+    r->user[0] = '\0';
+    size_t host_len = colon - dest;
+    if (host_len >= sizeof(r->host)) return -1;
+    memcpy(r->host, dest, host_len);
+    r->host[host_len] = '\0';
+  }
+  return 0;
+}
+
+Client *client_connect_ssh(char *destination) {
+  RemoteDest r;
+  if (parse_remote_dest(destination, &r) != 0) {
+    fprintf(stderr, "Invalid remote destination: %s\n", destination);
+    exit(EXIT_FAILURE);
+  }
+
+  int sv[2];
+  if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) < 0) {
+    perror("socketpair failed");
+    exit(EXIT_FAILURE);
+  }
+
+  int exec_pipe[2];
+  if (pipe(exec_pipe) < 0) {
+    perror("pipe failed");
+    exit(EXIT_FAILURE);
+  }
+
+  pid_t pid = fork();
+  if (pid < 0) {
+    perror("fork failed");
+    exit(EXIT_FAILURE);
+  }
+
+  if (pid == 0) {
+    close(sv[0]);
+    close(exec_pipe[0]);
+    fcntl(exec_pipe[1], F_SETFD, FD_CLOEXEC);
+
+    if (sv[1] != STDIN_FILENO)
+      dup2(sv[1], STDIN_FILENO);
+    if (sv[1] != STDOUT_FILENO)
+      dup2(sv[1], STDOUT_FILENO);
+    if (sv[1] > 1) close(sv[1]);
+
+    char ssh_user[512];
+    if (r.user[0] != '\0')
+      snprintf(ssh_user, sizeof(ssh_user), "%s@%s", r.user, r.host);
+    else
+      snprintf(ssh_user, sizeof(ssh_user), "%s", r.host);
+
+    execlp("ssh", "ssh", "-o", "Compression=no", "-o",
+           "ControlMaster=no", ssh_user, "fastsync-server", "--stdio",
+           (char *)NULL);
+    perror("exec of ssh failed");
+    (void)write(exec_pipe[1], "x", 1);
+    _exit(1);
+  }
+
+  close(sv[1]);
+  close(exec_pipe[1]);
+
+  char exec_status;
+  ssize_t n = read(exec_pipe[0], &exec_status, 1);
+  close(exec_pipe[0]);
+
+  if (n > 0) {
+    close(sv[0]);
+    waitpid(pid, NULL, 0);
+    fprintf(stderr, "Error: could not launch 'fastsync-server --stdio' on remote\n");
+    exit(EXIT_FAILURE);
+  }
+
+  Client *client = malloc(sizeof(Client));
+  client->file_descriptor = sv[0];
+  client->address.sin_family = AF_UNIX;
+  client->address_length = 0;
+  client->ssh_child_pid = pid;
+  return client;
+}