Function io_error_preserves_source_chain

atomic-core/src/pristine/error.rs:476–483

()

    #[test]
    fn io_error_preserves_source_chain() {
        let original = std::io::Error::new(std::io::ErrorKind::PermissionDenied, "access denied");
        let err: PristineError = original.into();

        // The source chain should be accessible for logging/debugging.
        let source = err.source().expect("Io variant should expose source");
        assert!(source.to_string().contains("access denied"));
    }

    #[test]
    fn custom_variants_have_no_source() {