Function equivalent_within 

pub fn equivalent_within(
    a: &SpatialProgram,
    b: &SpatialProgram,
    inputs: &[Inputs],
    rel: f64,
) -> Result<(), Divergence>

equivalent_under, but comparing floats within rel RELATIVE tolerance (falling back to rel as an absolute bound near zero, so a divergence cannot pass merely because both sides are small). Integers are still compared exactly — an approximate claim is about a float datapath, never about an integer one.