Multi-proofs: first pass at design

This issue is to track that we're doing some thinking and discussion about the design of the aggregate dispute game (multi-proofs).

No files, tests, or entry points are named. Start by reading the issue description and repository context for the aggregate dispute game (multi-proofs); done is not defined beyond completing the design discussion, so implementation scope must be established first.

Written by the indexing model from the issue text.