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Subresource Integrity is the standard for that https://www.w3.org/TR/SRI/

Do you know if this allows consolidation of multiple copies of the same resource from different hosts?

I don't know. At a guess such optimization would be beyond the scope of the standard - hence up to each vendor/browser/user-agent.

Not currently, because it conflicts with CSP. See https://news.ycombinator.com/item?id=10311020