N-Queens

Write `solve_n_queens(n)` returning every board on which `n` queens share no row, column, or diagonal. Each board is a list of `n` strings using `Q` and `.`. Place one queen per row, so rows never conflict by construction. Columns need a set. The trick is the diagonals: every cell on a down-right diagonal has the same `r - c`, and every cell on a down-left diagonal has the same `r + c`. That turns the check into two set lookups instead of a scan.