We already know the Mariners can make life hard on their pitching staff with the bats. That has been part of the deal for years. But this year, the problem has an extra layer. The Mariners are not just failing to back their pitchers with enough offense. They’re also not backing them defensively, and for a team that keeps selling itself on run prevention, this is pretty uncomfortable. If the whole idea is to build around pitching, defense, and clean baseball, then the defense can’t afford to be this shaky. It definitely cannot be sitting near the bottom of the league in Fielding Run Value while the rotation is still being asked to carry the weight of the entire operation. According to Baseball Savant’s Fielding Run Value leaderboard, the Mariners are at minus-11, ranking 29th among MLB teams. Only the Angels were worse, which feels especially rich considering Jo Adell robbed the Mariners of three home runs by himself. And the split plays an important role.…