scanKeys
fun scanKeys(store: DocumentStore, reader: IndexReader, matches: (Set<IndexTerm>) -> Boolean, present: (Boolean) -> Boolean = { it }): List<Key>
Every key a full scan says matches, with no index involved at all.
The other half of every differential test in the suite, and the fallback whenever the index cannot answer. Public because "what would a scan have said" is the question anybody debugging an index result actually has.