Redundant FK on parent relationship is denormalization?
Yes, it is denormalization. Any time you're copying data from one location in a database to another location (where the data could easily be obtained from its original location), it's denormalization by definition.

And, improving performance is a perfectly good reason to denormalize. That said, unless you have a huge number of clients, or the client_id is the only data from the sale table that you would normally need, then it shouldn't be a major concern. It's entirely reasonable to cause this particular instance of denormalization "cascading key structure", or anything else appropriate. Doesn't mean it's not also denormalization.

And, with respect to not having consistency issues - never say never....