The Mariners have reached the rarest kind of pitching problem, which is also the easiest one to mock until a team actually has it. Too many starters. That’s why Seattle is preparing to test out a six-man rotation, at least for now. Bryce Miller is expected back from the injured list and will make his first start on May 13. Emerson Hancock did more than keep the seat warm while Miller was out. And Luis Castillo’s struggles have complicated the clean version of the plan.  So, this is a bit of luxury. But it’s also a little bit of a mess. We say “you can never have too much pitching,” and we mean it, because baseball has a way of decimating rotations. But there’s still a difference between having depth and managing it.  A six-man rotation sounds simple on paper. Just add another guy. In reality, it asks everyone to bend a little. And that’s what made Woo’s response so important. Woo told MLB.com that the Mariners’ surplus of starting pitching talent is a “privilege,” which is the right place to start.…