Fix final sigma handling in :greek

[AlexGx]

Final sigma casing is incorrect in several common cases

Current implementation checks already lowercased output acc instead of the original input context

iex> String.downcase("AΣ", :greek)
"aσ" # expected "aς"

ex> String.capitalize("ΑΣ", :greek)
"Ασ" # expected "Ας"

Final sigma applied when:

• has preceding cased character, ignoring case-ignorable characters
• does not have a following cased character, again ignoring case-ignorable characters

Cases covered:

• cased characters from Latin and Greek scripts
• case-ignorable punctuation and combining characters
• Other_Lowercase / Other_Uppercase Unicode props
• expanded lowercase mappings such as İ
• capitalization after cased / uncased prefixes ("AΣ" → "Aς", "1Σ" → "1σ")

(Reviewed) Assisted-by: GPT-5.6 Sol

[josevalim]

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