Longest Digit Run

**Description:** Given a positive integer n, find the length of the longest run of equal consecutive digits (in normal left-to-right order). **Input:** One integer n (1 ≤ n ≤ 10^18). **Output:** One integer — the longest run length. **Example:** 1223331 → the run 333 has length 3. *Note:* digits arrive right-to-left from % 10 — track the current run carefully as digits change.