Function data_dir 

pub fn data_dir() -> PathBuf

Returns the data directory following XDG spec.

Uses XDG_DATA_HOME if set, otherwise ~/.local/share/runtimo.

Falls back to /tmp/runtimo with a stderr warning when neither XDG_DATA_HOME nor HOME is set. Data in /tmp is not persistent across reboots — WAL and backup durability guarantees are degraded in this fallback mode.