Two Stats, One Pass

Use Collectors.teeing (or an equivalent single-pass reduction) on a list of integers: - static double average(List<Integer> xs) — arithmetic mean; empty throws IllegalArgumentException. - static Map<String, Integer> rangeStats(List<Integer> xs) — returns {"min": ..., "max": ...} computed in one teeing pass. - static String classify(List<Integer> xs) — stream composition drill: map each x to x*2, keep only values > 10, join with commas; return the joined string (empty list → empty string).

Difficulty: intermediate