Find kth smallest in sorted 2D matrix

You are given an `n x m` matrix of integers where **each row and each column is sorted in non-decreasing order**. You are also given an integer `k` such that `1 <= k <= n * m`. Return the **k-th smallest element** in the matrix, where elements are ordered by value in non-decreasing order and duplicates are counted according to their occurrences. **Example:** - Input: - `matrix = [[1, 5, 9], [10, 11, 13], [12, 13, 15]]` - `k = 8` - All elements in sorted order: `[1, 5, 9, 10, 11, 12, 13, 13, 15]` - Output: `13` (the 8th element in the sorted list). Design an algorithm that is more efficient than flattening the matrix and fully sorting all `n * m` elements.