Equivalent to https://schema.org/contributor

I haven't seen co-authorship addressed at either https://indieweb.org/authorship nor https://indieweb.org/authorship-spec with any formalization. Please align with schema.org's https://schema.org/contributor specification?

Start by reviewing the authorship and authorship-spec pages linked in the issue, then compare their current treatment of authorship with schema.org/contributor. Define the missing co-authorship behavior and document a precise, agreed specification; done means the project’s formalization explicitly addresses contributor relationships.

Written by the indexing model from the issue text.