Case-map İ and ı, which the invariant mapping leaves alone [patch]

[matt-edmondson]

Fixes #97

What changed

.NET's invariant casing deliberately leaves U+0130 İ and U+0131 ı unmapped. As a result, snake, kebab and camel output kept an uppercase İ, and MACRO and Pascal output kept a lowercase ı.

• Case-mapping helpers. ToUpperInvariantFull and ToLowerInvariantFull apply the invariant mapping, then patch those two letters to their Unicode simple mappings (ı → I, İ → i). A culture TextInfo is not used, because its result depends on the installed ICU data. Every casing call goes through the helpers:

  • snake/macro conversion
  • the first-char helpers
  • the possessive-suffix uppercasing
  • IsLowercaseWithNoUppercase. Before this change ı counted as "no uppercase form", which also made "ı".IsAllCaps() return true.

• Title case. TitleCaseKeepingTypographicApostrophes swaps İ/ı for I/i before the invariant TextInfo.ToTitleCase runs. It then reads each result back:

  • an I means the letter starts a word, so İ is kept and ı becomes I
  • an i means it does not, so İ becomes i and ı is kept

  This is what makes "ılık su".ToPascalCase() give IlıkSu. "İZMİR" normalizes to İzmir, the same as "İzmir" does.

Call	Before	After
"İzmir city".ToSnakeCase()	İzmir_city	izmir_city
"İzmir city".ToCamelCase()	İzmirCity	izmirCity
"İstanbul".ToLowercaseFirstChar()	İstanbul	istanbul
"kırmızı".ToMacroCase()	KıRMıZı	KIRMIZI
"ılık su".ToPascalCase()	ılıkSu	IlıkSu
"ılık su".ToUppercaseFirstChar()	ılık su	Ilık su

Out of scope

ToSnakeCase still lowercases the ToMacroCase output, so a lowercase ı round-trips to i in snake/kebab case ("kırmızı".ToSnakeCase() → kirmizi). The camel path has the same problem for a leading ı. That upper→lower round trip is #84, so these tests don't assert those outputs.

Testing

• New tests: data rows for İ across all five converters, ı rows for Pascal and Macro, the first-char helpers and ToTitleCase, ToMacroCase idempotence on kırmızı / İzmir city / ılık su, and IsAllCaps on ı.
• With the source change reverted, 6 tests fail. With it, all 94 pass on net10.0.
• dotnet build CaseConverter.sln builds every target (net10.0/9.0/8.0, netstandard2.0/2.1) with 0 warnings.

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https://claude.ai/code/session_01AtF6Ynq3K6Km6HSRHaKuX8

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
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100.0% Coverage on New Code
0.0% Duplication on New Code

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