Validate a 9×9 grid under constraints

## Problem You are given a 9×9 grid representing a partially filled puzzle. Each cell contains either: - a digit character `'1'`–`'9'`, or - the character `'.'` meaning empty. Determine whether the current filled cells are **valid** under these rules: 1. No digit `'1'`–`'9'` repeats within any row. 2. No digit repeats within any column. 3. No digit repeats within any of the nine 3×3 sub-grids. Return `true` if the filled cells satisfy all rules; otherwise return `false`. ## Input / Output - **Input:** `board` — a 9×9 array of characters - **Output:** boolean ## Notes / Constraints - You only need to validate the currently filled cells; you do **not** need to solve the puzzle. - The board is always 9×9. ## Example If a row contains two `'5'` values (ignoring `'.'`), the board is invalid and you should return `false`.