Confidence must be attributed, never a mark

Layer: assessment.

From the mark discipline in kb/types/tag-readme.md (ADR 026): a mark is a cache recomputable from ground truth and validated by code. A credence is not recomputable — a bare confidence: 0.7 field would be exactly the "stale trusted cache is a trap" failure the mark rule forbids. Represent credences only as attributed, sourced assessments ("Rootclaim's aggregate: 96% zoonotic") — claims about who-believes-what, not KB-blessed truth.

This is why bare confidence fields (confidence: low-to-medium, extraction_confidence: high) are rejected — see rejected-candidates.