Starlark-based build files and target graph

Fabrik now reads per-package fabrik.star files, walks the workspace to discover them, and provides a glob() helper that expands patterns relative to the package directory. You can declare rust_binary, rust_library, and rust_test targets, run fabrik targets to list every target by label, and run fabrik run //pkg:name to build a Rust binary through the action cache into .fabrik/out/<package>/<name>.