Grounding faithfulness is measured on two axes: whether the answer follows the retrieved context, and whether each claim is supported by the source it cites. Both can be satisfied in full by an answer that picks one side of a genuinely contested question, cites a source that holds that side, and drops the credible sources that disagree. We name this failure disagreement collapse, or the citation-faithfulness illusion, and treat it as a construct-validity hazard: a high citation score does not license the inference that the answer reflects the sources. Conflict-aware retrieval already prescribes reflecting credible debate; our contribution is to make that prescription measurable. We define disagreement recall and disagreement-attribution precision, both decoupled from answer correctness and conditioned on source credibility so that suppressing a debunked claim is not penalized; a five-component rubric with verbatim anchors; an annotation protocol for the credibility filter and for position identity; a matched-pair diagnostic that holds retrieval and citation availability fixed; and a pre-registered two-judge pilot with human adjudication. No empirical scores are reported; the contribution is the instrument and its frozen evaluation design.