Membership with many role leaders

We might want to offer leaders be in various roles.

`leader(of: role) -> Member?`

We don't yet have member roles, and they should be dynamic as well;
So we'd want to be able to change a member's role and run an election for that role.
This might want to require the "all members have seen the role change" and similar requirements which we're still getting away without... 🤔

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Do we want to have some capability for "2nd in line leaders"? Just random idea... let's not do it right now perhaps, though food for thought.

No file, test, or entry point is named. Start by locating the existing membership and leader-election design, then clarify the role model, role changes, and visibility requirements before defining what an accepted implementation would include.

Written by the indexing model from the issue text.