tatara-rust-test — L3 scaffolding.
Given a typed [tatara_rust_derive::ProcDeriveSpec], materialize a
test environment on disk that exercises the generated proc-macro
end-to-end:
- emit the derive crate to a temp dir
- emit a minimal consumer crate that depends on it
- exposes paths for an external driver to run
cargo test
Snapshot-friendly: [as_files] returns the in-memory file map for
insta::assert_yaml_snapshot!.