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/shared/socket.h

@@ -21,13 +21,16 @@ typedef struct Client {
   struct sockaddr_in address;
   unsigned int address_length;
   int file_descriptor;
+  pid_t ssh_child_pid;
 } Client;
 
 Client *client_create();
 void client_disconnect(Client *client);
 void client_delete(Client *client);
 void client_connect(Client *client, char *host, int port);
+Client *client_connect_ssh(char *destination);
 
+void io_set_fds(int read_fd, int write_fd);
 void send_n_data(int file_descriptor, void *data, size_t data_size);
 void receive_n_data(int file_descriptor, void *data, size_t data_size);
 void send_str(int file_descriptor, char *data);