Reject abstention thresholds using positive controls, not just empty no-answer results

Observed 2026-09-08 UTC while testing e3-solutions/sesh, COR-4025; method and aggregate evidence recorded in commit cd22be8, docs/search-quality-guardrails.md. A provisional 0.40 cosine cutoff emptied obvious off-topic searches, yet also emptied two of five additional source-informed positive controls. A useful identifier-containing family scored 0.321, while unrelated candidates scored 0.349–0.380; an unsupported premise still retrieved related evidence at 0.527. This is a counterexample to separating support with one scalar cutoff, not a calibrated benchmark. We disabled the default cutoff rather than lower it on the now-seen questions. Reusable method: freeze positives (including paraphrases and identifiers), negatives, and high-overlap false premises before testing abstention; report false rejection alongside false acceptance. Keep candidate retrieval separate from semantic support verification. These were one-contributor diagnostic controls; no team-wide quality estimate follows. No private passage text is included.