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

tests/test_config.c

@@ -18,6 +18,20 @@ static void test_config_lifecycle() {
   EXPECT_FALSE(cfg->use_chunk_serialization);
   EXPECT_FALSE(cfg->use_compression);
   EXPECT_EQ_INT(cfg->num_connections, 4);
+  EXPECT_EQ_INT(cfg->transport, TRANSPORT_TCP);
+  EXPECT_NULL(cfg->ssh_destination);
+  config_delete(cfg);
+}
+
+static void test_config_ssh_dest() {
+  Config *cfg = config_create(str_dup("1.0"), str_dup("/src"), str_dup("user@host:/dst"),
+                              true, false, false, false, false, 1, 4, false);
+  cfg->transport = TRANSPORT_SSH;
+  cfg->ssh_destination = str_dup("user@host:/dst");
+  EXPECT_NOT_NULL(cfg);
+  EXPECT_EQ_INT(cfg->transport, TRANSPORT_SSH);
+  EXPECT_EQ_STR(cfg->ssh_destination, "user@host:/dst");
+  EXPECT_EQ_STR(cfg->receive_root_directory, "user@host:/dst");
   config_delete(cfg);
 }
 
@@ -55,6 +69,7 @@ static void test_pipeline_receiver_lifecycle() {
 
 void test_config() {
   test_config_lifecycle();
+  test_config_ssh_dest();
   test_pipeline_sender_lifecycle();
   test_pipeline_receiver_lifecycle();
 }