Route Fabrik state through XDG Base Directory paths

Build state now lives in standard XDG directories instead of inside the workspace. The content-addressed cache is stored at $XDG_CACHE_HOME/fabrik/cas, the Elixir compile daemon socket at $XDG_RUNTIME_DIR/fabrik, and the materialized daemon script at $XDG_DATA_HOME/fabrik/daemon. Build outputs remain in <workspace>/.fabrik/out. This lets identical actions cache-hit across different projects on the same host and lets you redirect all state via the usual XDG environment variables.