The veneer field on a library in librarian.yaml is a top-level boolean that indicates a library has handwritten code wrapping generated code. In practice, only Rust uses this field at runtime, and every Rust veneer library already has rust.modules configured, so this field is redundant.

We should replace the veneer field with a derived method IsVeneer() that returns true when language-specific module config is present (i.e., lib.Rust != nil && len(lib.Rust.Modules) > 0)