Move Fabrik state to XDG Base Directory paths

Fabrik state now follows the XDG Base Directory specification, so identical actions hit across projects on the same host instead of duplicating per workspace: blobs, action results, and scratch live under $XDG_CACHE_HOME/fabrik/cas; the elixir daemon socket lives under $XDG_RUNTIME_DIR/fabrik (with a uid-keyed tempdir fallback); the materialized daemon script lives under $XDG_DATA_HOME/fabrik/daemon. Build outputs and runtime sessions stay in <workspace>/.fabrik/out, where users consume their artifacts. dispatch now also validates that -C <path> resolves to a directory.