fix: raise ValueError in encode() for non-lowercase input

[alisatwat3]

encode() previously accepted uppercase letters, digits, and other non-lowercase characters silently, producing incorrect/out-of-range values (e.g. negative numbers for uppercase letters) instead of failing. Add input validation using str.islower() and str.isalpha() to raise a ValueError when the input isn't purely lowercase a-z. Added a doctest covering the new error case.

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• Fix a bug or typo in an existing algorithm?
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