Input: a single Reference (already deserialized); output: a Wikitext string. (“Service” probably means something like “an interface, a class implementing it which gets other ‘services’ as constructor parameters, and a getter on WikibaseClient”.)
This service might live in Lib or Client; that probably depends on the other services it gets injected. If Lib is possible, we would prefer that.
The reference style is not part of this interface; for now, we only support a single style anyways.