You're given an m × n matrix where every row is sorted ascending left to right, and every column is sorted ascending top to bottom — but unlike the simpler matrix search, it does not flatten into one sorted sequence. Given an integer target, return whether it exists anywhere in the matrix. Solve it in O(m + n) time using a staircase search that starts at the top-right corner and, on each comparison, eliminates an entire row or an entire column.