Matrix — Valid Sudoku

solve(board) — board is a 9×9 grid of strings: "1"–"9" for a filled cell and
"." for an empty one. Return true when the filled cells break no Sudoku rule:

• each row holds no digit twice;
• each column holds no digit twice;
• each of the nine 3×3 boxes holds no digit twice.

You are *not* asked to solve the puzzle or to check it can be completed — only that what
is on the board is consistent. Do it in a single pass over the 81 cells with one set per
row, per column and per box. The box of cell (r, c) is floor(r / 3) * 3 + floor(c / 3).

Examples

solve(board with 5 3 . . 7 . . . . on the first row …)  → true
solve(same board, but the top-left cell is "8")         → false   // 8 repeats in column 0 and in the box