No, but it’s planned; probably in the form of implicit parameters.

Is there a fundamental limitation that would make it impossible to use a haskell-style coherent typeclass system, where the instance of a typeclass must be globally unique?

My guess is yes, since that would probably forbid modifying the instance, since we would now have both the new and old copy. Unless of course we couple type class instances with the definition of the type itself (or the definition of the class) and view the type together with all its instances as a unit (as Unison does with mutually recursive functions), but that would bring on its own set of issues.