The Mariners’ rotation question is starting to feel less like a depth problem and more like a truth serum. That is usually what happens when too many arms have a claim to too few spots. But there’s still a difference between having depth and having clarity. Right now, the Mariners have plenty of the first one. The second one is where things get messy. Adam Jude of The Seattle Times recently floated a piggyback scenario as one possible way Seattle could handle the squeeze, with Castillo and Miller potentially sharing a game instead of both being forced into a traditional rotation lane. That’s the kind of idea that makes you stop for a second. Luis Castillo and Bryce Miller in a piggyback setup? One starts, one follows, and suddenly the Mariners are treating two established rotation arms like a tandem project. At first, it sounds like a little much. Then you look at the situation again and realize it might actually be the cleanest messy answer available. The Mariners don’t need this to be a long-term solution.…