Function check_pixel_roundtrip 

pub fn check_pixel_roundtrip<E, D>(
    enc: E,
    dec: D,
    img: &TestImage,
) -> Conformance
where
    E: EncoderConfig,
    D: DecoderConfig,
    <E::Job as EncodeJob>::Enc: Encoder<Error = E::Error>,

A round trip through one-shot encode → one-shot decode reproduces the input pixels exactly. The smallest sanity check; a failure here means nothing else is trustworthy.