Erasure

MemberData::export(member) lists what the collections keep of a member: their copies, the sets they completed, the packs they opened with their seeds, and the dice they chose, each as plain values.

erase(member) deletes all of it and returns an Erasure with the count of each. The copies the member traded away belong to other members and stay theirs: a copy remembers its source, never the members it went through. Their activity is erased by tilttek/activity, and their offers by tilttek/exchange.