Function latest_session_dir 

pub fn latest_session_dir() -> PathBuf

Resolve the parent directory the runtime treats as the session root when --session-path is absent. See default_session_dir for the three-case logic. <root>/latest — the symlink the runtime maintains to point at the most recent session dir. Read-side commands (nmbrs report, nmbrs plot, nmbrs replay, …) default to reading through this path so “show me my latest run” is the natural no-arg invocation.