Z-Score Standardization

Standardizing recentres a feature on zero and rescales it so one unit means one standard deviation:

Task: write zscore(values) returning the standardized list, rounded to 4 decimal places.

• Use the population standard deviation — divide the squared deviations by n, not n - 1.
• If every value is identical the deviation is zero; return all zeros rather than dividing by zero.
• An empty list returns an empty list.

Unlike min-max scaling this has no fixed range, which is exactly why outliers hurt it less — an extreme value lands at z = 8 and stays out there instead of crushing everything else toward one end.