isUniqueKey

Whether key appears in exactly one usable segment.

Decided from the key blocks, which is a sidecar read: BaseSidecar.ordinalOf is a bisect over mapped bytes and opens no document. That is what lets a caller skip a recheck it can prove is a no-op without weakening the rule that every hit is rechecked — a key living in one segment has one version, so the column's value is the visible value.