feat: add SSH transport, rsync-style CLI, and --stdio server mode

- Replace send()/recv() with read()/write() + io_set_fds() for fd abstraction
- Add client_connect_ssh() via socketpair + fork/exec; reap child in client_disconnect()
- Add server --stdio flag for SSH invocation
- Replace handle_arg() with rsync-style positional args (fastsync <src> <dest>)
- Add SSH vs TCP auto-detection via is_remote_dest()
- Add TransportType and ssh_destination to Config, fix uninitialized fields in
  config_receive() (pre-existing multithreaded crash)
- Sender threads now wait for server STATUS_OK before disconnecting
- Add --help, error handling for sendfile+SSH combos
- Add pre-flight checks and Posix Args test case to test.py
- Add test_config_ssh_dest() unit test

src/server/server.c

@@ -10,6 +10,7 @@
 #include "utils.h"
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <threads.h>
 
 File *file_receive(Config *config, int file_descriptor) {
@@ -161,12 +162,18 @@ void handler(int file_descriptor) {
     thrd_join(receiver, NULL);
     thrd_join(writer, NULL);
     pipeline_context_receiver_destroy(context);
+    send_status(file_descriptor, STATUS_OK);
   } else
     receive_files(config, file_descriptor);
   close(file_descriptor);
 }
 
-int main() {
+int main(int argc, char *argv[]) {
+  if (argc > 1 && strcmp(argv[1], "--stdio") == 0) {
+    io_set_fds(STDIN_FILENO, STDOUT_FILENO);
+    handler(STDIN_FILENO);
+    return 0;
+  }
   Server *server = server_create(8080);
   server_listen(server, handler);
   server_delete(server);