Single-threaded. The Wren VM itself is completely self-contained and has no global state. So, if you wanted to, you could host multiple VMs simultaneously, one for each thread, to get actual concurrency. You'd have to do the marshalling to move values from one to the other to get sharing or communication working, though.
If the VMs are self-contained, what mechanism would they use to communicate? id be ok to use marshaling if the marshaling itself is the dominating cost of communication.
That's up to the program embedding them -- the idea is that you're the one implementing the communication and synchronization in the embedding program, rather than in Wren itself.
