Ignore case for letters outside the Basic Multilingual Plane [patch]

[matt-edmondson]

Fixes #90

What was wrong

Fuzzy.CodepointsEqual compared a surrogate pair one code unit at a time. Letters outside the Basic Multilingual Plane, such as Deseret, Osage and Adlam, therefore matched only in the exact case, although the README says case is always ignored.

Change

• Comparison: a surrogate pair is now compared as one codepoint in the new SurrogatePairsEqualIgnoringCase. Like CharsEqualIgnoringCase (Case-insensitive matching fails for Greek final sigma and micro sign: Contains("ΟΔΟΣ", "οδος") is false #88), it matches on either the lowercase or the uppercase form.
  • On .NET Core 3.0 and later it uses Rune.ToLowerInvariant / Rune.ToUpperInvariant.
  • On netstandard2.0 and netstandard2.1, where Rune doesn't exist, it falls back to invariant string casing of the two-character pair.
• Remark: the XML remark on CodepointsEqual now gives the real reason a pair has to be compared whole. It no longer says supplementary codepoints are compared exactly.

Not in this PR

The camelCase bonus still reads case from the lead surrogate, so a supplementary-plane uppercase letter does not earn it. The issue lists this as optional. Fixing it changes the internal ApplyBonuses(…, char strChar, char strLower, char strUpper, …) signature that the existing tests call directly, so it is better as its own change. It does not affect the acceptance criteria: the other-case match now scores the same as the exact-case one.

Tests

New tests in the Surrogate Pair region:

• Deseret and Adlam in both directions, plus a two-letter Osage case
• Each other-case score equals the exact-case score
• Two different Deseret letters still don't match

Results:

• Before the fix, 8 of the new cases fail.
• After the fix, the full suite passes: 74 of 74 on net10.0, the test project's only target.
• The library builds clean, with 0 warnings, for net10.0, net9.0, net8.0, netstandard2.0 and netstandard2.1.
• The netstandard fallback is compiled but not run by the tests, because there is no netstandard test leg.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QnUaFtfBEaU4WcCPkkdpvM

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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