Function check_metadata_rewrite 

pub fn check_metadata_rewrite<'a>(
    request: RewriteRequest<'a>,
    rewrite: impl Fn(&RewriteRequest<'a>) -> Result<Plan<'a>, Error>,
    observe: impl Fn(&[u8]) -> Conformance,
) -> Conformance

Exercise a codec’s metadata-only rewrite entry point with caller-owned fixtures. The observer must independently verify image payloads, timing, rendering metadata and requested retention (prefer a third-party reader). Checks plan materialization, report ranges, deterministic output, typed input, output and carrier limits, and pre-cancellation. Services are retained.