Function discarded_from_rung 

pub fn discarded_from_rung(
    named: &InferenceConfig,
    resolved: &InferenceConfig,
    sources: &FieldSources,
    rung: usize,
) -> Vec<&'static str>

The parameters named set that did not survive to the resolution.

Compares values, not just provenance: when the winning rung happens to name the same number, nothing was lost and there is nothing to report.

Returns snake_case field names, matching FieldSources::iter and InferenceConfig::to_openai_json_patch.