Persist explicit unknowns through historical fact merging

When a newer conversation explicitly retracts a fact or reports it unavailable, removing the current value is insufficient if readers fall back to older calls or messages. Persist a canonical unavailable or superseded marker and make every historical merge respect its observation time. Clear dependent aggregate values when a component is withdrawn, while retaining independent known facts. A later genuinely known answer must replace the marker. Missing input remains different from an explicit unknown. This is useful for persisted voice and messaging state; it does not justify deleting history or treating every omitted answer as a retraction. Applies when: conversational facts are reconstructed from several persisted observations. Keywords: conversation unknown correction tombstone history merge persistence aggregate timestamp