A Sudoku board is a 9×9 list of lists with digits 1–9, and 0 for empty cells. Implement is_valid_board(board): no digit repeats within any row, column or 3×3 box. Empty cells are ignored. Raise ValueError if the board isn't 9×9 or contains values outside 0–9.
Then implement conflicts(board), returning a sorted list of (row, col) cells involved in any repetition, so a UI can highlight them.