Minimax elevation path

You are given an n x n `grid` of distinct elevations. Starting at the top-left cell (0,0), you want to reach the bottom-right cell (n-1, n-1). You may move up/down/left/right. The cost of a path is the maximum elevation of any cell it visits (including start and end). Return the minimum possible path cost — i.e. the smallest value t such that there's a path where every cell's elevation is <= t.