Thanks for the feedback, a couple of random updates and comments:
Just to clarify the discussion about "actor groups" above, I am not in favor of introducing another abstraction here. I think that actors by themselves are enough.
I still think it makes sense for ActorSendable to be distinct from Codable, because certain things are actor sendable but may not be codable, e.g. an internally synchronized reference type. However, it could make sense to make ActorSendable, Codable, and ValueSemantic refine each other. I think it would be best to discuss this in the ValueSemantic thread.
Also, I forgot to mention how closures / function types fit into this. They are a bit weird because they aren't nominal types, but we should be able to support them. I added a section to future work to describe the approach I recommend. I welcome comments and thoughts as always.
-Chris