Fabrik state now uses XDG Base Directory paths

Fabrik now stores its content-addressed cache under $XDG_CACHE_HOME/fabrik/cas, the Elixir compile daemon socket under $XDG_RUNTIME_DIR/fabrik, and the materialized daemon script under $XDG_DATA_HOME/fabrik/daemon. This shares identical build actions across projects on the same host and follows standard conventions for application data.