Function connection_file_path 

pub fn connection_file_path() -> PathBuf

Resolve subc’s per-user TCP connection-file path.

$XDG_RUNTIME_DIR/subc-connection.json is preferred because the runtime directory is already per-user on Unix desktops. Without it, subc falls back to the system temp dir with a per-user token in the filename so different OS users do not collide on shared temp directories.