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/config.h

@@ -3,6 +3,11 @@
 
 #include <stdbool.h>
 
+typedef enum {
+  TRANSPORT_TCP,
+  TRANSPORT_SSH
+} TransportType;
+
 typedef struct Config {
   char *version;
   char *send_directory;
@@ -16,6 +21,8 @@ typedef struct Config {
   bool use_metadata;
   int compression_level;
   int num_connections;
+  TransportType transport;
+  char *ssh_destination;
 } Config;
 
 Config *config_create(char *version, char *send_directory,