Add polyfills for char.IsBetween(...) and char.IsAscii*(...)

[Copilot]

Polyfills for the char static methods introduced in .NET 7, making them available on older targets (netstandard, netframework, pre-NET7 netcoreapp).

New polyfills — Net70/Char.cs

Method	Notes
char.IsBetween(c, min, max)	unsigned subtraction trick
char.IsAscii(c)	(uint)c <= 0x7f
char.IsAsciiDigit(c)	'0'–'9'
char.IsAsciiLetter(c)	upper or lower
char.IsAsciiLetterUpper(c)	'A'–'Z'
char.IsAsciiLetterLower(c)	'a'–'z'
char.IsAsciiLetterOrDigit(c)	letter or digit
char.IsAsciiHexDigit(c)	upper or lower hex
char.IsAsciiHexDigitUpper(c)	'A'–'F' or digit
char.IsAsciiHexDigitLower(c)	'a'–'f' or digit

// Available on netstandard / netframework / pre-.NET 7
char.IsAscii('A');                        // true
char.IsAsciiDigit('9');                   // true
char.IsAsciiHexDigit('f');                // true
char.IsBetween('m', 'a', 'z');           // true
char.IsAsciiLetterOrDigit('!');          // false

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.15%. Comparing base (3bf818d) to head (41d9ccc).

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #129      +/-   ##
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+ Coverage   99.14%   99.15%   +0.01%     
==========================================
  Files         143      145       +2     
  Lines        3727     3798      +71     
  Branches       87       87              
==========================================
+ Hits         3695     3766      +71     
  Misses         13       13              
  Partials       19       19              

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[Copilot]

Pull request overview

Adds .NET 7 char static API polyfills (ASCII classification helpers and IsBetween) so consumers targeting older TFMs (netstandard, netframework, pre-.NET 7 netcoreapp) can call the newer char.* methods with the same surface area.

Changes:

• Introduces Net70/Char.cs polyfills for char.IsBetween and the char.IsAscii* family.
• Adds PolyShim.Tests/Net70/CharTests.cs to validate the new polyfilled APIs across the test TFMs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
PolyShim/Net70/Char.cs	Adds .NET 7 char polyfill members (IsBetween / IsAscii / IsAscii*) behind appropriate TFM guards.
PolyShim.Tests/Net70/CharTests.cs	Adds unit tests covering the new char polyfills’ expected true/false cases.

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[Tyrrrz]

@copilot some methods were introduced in .NET 6

[Copilot]

@copilot some methods were introduced in .NET 6

Moved char.IsAscii to a new Net60/Char.cs (guarded by !NET6_0_OR_GREATER) since it was introduced in .NET 6. The remaining methods (IsBetween, IsAsciiDigit, IsAsciiLetter*, IsAsciiHexDigit*) stay in Net70/Char.cs as they were all introduced in .NET 7. The test was moved to Net60/CharTests.cs accordingly. (commit $(git rev-parse --short HEAD 2>/dev/null || echo "latest"))

@copilot some methods were introduced in .NET 6

Moved char.IsAscii to a new Net60/Char.cs (guarded by !NET6_0_OR_GREATER) since it was introduced in .NET 6. The remaining methods (IsBetween, IsAsciiDigit, IsAsciiLetter*, IsAsciiHexDigit*) stay in Net70/Char.cs as they were all introduced in .NET 7. The corresponding test was moved to Net60/CharTests.cs as well.