The Luis Castillo problem is not going away because this no longer looks like one bad start. The Mariners can call it a rough stretch, and maybe that is all it becomes, but it’s already big enough to force an uncomfortable conversation. Castillo is struggling, Bryce Miller is working his way back, Emerson Hancock has complicated the rotation picture, and Seattle suddenly has a very expensive starter sitting at the center of a problem with no clean answer. The Mariners are dealing with a rotation dilemma that has money, reputation, roster flexibility, injured-list timing, and actual performance all tangled together like a pair of headphones pulled from the bottom of a backpack. Castillo’s struggles are the headline. The problem underneath is much messier. After Seattle’s 11-4 loss to the Twins, it became even harder to pretend that this conversation is going away quickly. Castillo allowed seven earned runs in five innings against Minnesota, and his rough stretch seems to have no end.…