Merge Duplicate Cluster

Fold a whole cluster into one survivor, one reversible fold per member.

The cluster is re-derived from live pairs before anything is written. A cluster is computed rather than stored, so the set the operator was looking at can have changed between the read and the click — a pair dismissed, a profile merged some other way, a side reclassified as a shared inbox. Acting on the stale set would merge people nobody agreed to merge, which is the same reason auto-merge re-verifies rather than trusting a materialized row.