# Compress Consecutive Characters with Run Lengths Implement `compress_runs(s)` and return the compressed string. The input contains ASCII letters only. Scan left to right. Emit a character unchanged for a run of length one; otherwise emit the character followed by its decimal run length. For example, `"aaaaabbbccca"` becomes `"a5b3c3a"`, and an empty string returns an empty string. Constraints: `0 <= len(s) <= 200000`; the encoded output contains at most `400000` ASCII characters. ```hint Check representation boundaries Include tests for an empty string, one character, a single long run, and a change after a multi-digit count. ```