fix: make camelToUpperSnake Unicode-safe for acronym boundaries

[masonkatz]

Summary

• iterate and look ahead by rune rather than UTF-8 byte
• add a Greek acronym-boundary regression test

Fixes #4

Validation

• make test
• make lint (currently reports five pre-existing goconst findings on origin/main)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.14%. Comparing base (8a905d7) to head (47871f1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
+ Coverage   78.06%   78.14%   +0.08%     
==========================================
  Files           8        8              
  Lines         269      270       +1     
==========================================
+ Hits          210      211       +1     
  Misses         55       55              
  Partials        4        4              

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