List Duplicate Clusters

Open pairs of one tier, closed transitively into review decisions.

One tier per request, never mixed: an exact component and a strong one joined together would be a cluster nobody can act on, because half of it is safe to merge and half needs a person to look.

The closure is computed once and paged in memory. Unlike the pair queue it cannot be paged in SQL — a component is only known once every pair has been walked — so _CLUSTER_COUNT_MAX bounds it and the count says so.