Valid Sudoku

Write `is_valid_sudoku(board)` for a 9x9 board given as a list of 9 lists of single-character strings, where `"."` marks an empty cell. Return `True` if no row, column, or 3x3 box contains a repeated digit. Only the filled cells are checked — the board does not have to be solvable, or even solved. Three separate passes work. One pass with three arrays of sets is better, and the only fiddly part is mapping a cell to its box: `(row // 3) * 3 + col // 3`.