Function compare_ends 

pub fn compare_ends(
    first: &FragmentEnd,
    first_side: EndSide,
    second: &FragmentEnd,
    second_side: EndSide,
) -> Result<EndCompatibility, CompatibilityError>

Compare physical oligos (each written 5′→3′). Enzyme names are provenance, not compatibility criteria. Same-side ends require reversing fragment two.