The Mariners didn’t need the Braves to accidentally help an AL West rival in order to be reminded how quickly catching depth can become a real issue. But, here we are. On May 4, the Athletics acquired veteran catcher Jonah Heim from Atlanta in exchange for cash considerations, a move that came after the Braves designated him for assignment to make room for Sean Murphy’s return from the injured list. The timing was almost too on the nose. Seattle is literally in the middle of a series against Atlanta, and while the Braves were clearing out their own catching surplus, the Athletics swooped in to add a familiar face behind the plate.   This isn’t a breaking headline moment that should spark fear into the Mariners fanbase. Heim is a useful veteran. He is a switch-hitter. He has caught winning baseball as part of the Rangers’ 2023 World Series team. He’s also been on a pretty real offensive slide since that peak, which is why he was available for cash in the first place.…