Add polyfill for Math.Log2(double)

[Copilot]

Math.Log2(double) was introduced in .NET Core 3.0 / .NET Standard 2.1 and had no polyfill for earlier targets.

Polyfill

• Added PolyShim/NetCore30/Math.cs with a static extension implementing Math.Log2(double) as Math.Log(x, 2), guarded to only apply on .NET Framework, pre-.NET Core 3.0, and pre-netstandard2.1 targets.

Math.Log2(8.0); // 3.0

Tests

• Added PolyShim.Tests/NetCore30/MathTests.cs covering typical values and edge cases (0, +Infinity, negative input, NaN).

Docs

• Regenerated Signatures.md to include the new member.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.17%. Comparing base (527ee10) to head (0085351).

Additional details and impacted files

@@           Coverage Diff           @@
##            prime     #136   +/-   ##
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  Coverage   99.17%   99.17%           
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  Files         149      150    +1     
  Lines        3998     4005    +7     
  Branches       88       88           
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+ Hits         3965     3972    +7     
  Misses         13       13           
  Partials       20       20           

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[Copilot]

Pull request overview

This pull request adds a PolyShim polyfill for System.Math.Log2(double) for targets that predate .NET Core 3.0 / .NET Standard 2.1, along with tests and signature documentation updates.

Changes:

• Added a NetCore30 polyfill for Math.Log2(double) implemented via an extension member.
• Added NetCore30 tests covering standard and edge-case inputs for Log2.
• Regenerated Signatures.md to include the new member and updated totals.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Signatures.md	Updates signature inventory totals and lists Math.Log2(double) as a .NET Core 3.0 member.
PolyShim/NetCore30/Math.cs	Introduces the Math.Log2(double) polyfill under appropriate target guards.
PolyShim.Tests/NetCore30/MathTests.cs	Adds test coverage for Math.Log2(double) including edge cases.

Suppressed comments (1)

PolyShim.Tests/NetCore30/MathTests.cs:18

• Asserting Be(double.NaN) can be brittle because NaN != NaN under == semantics. To make intent unambiguous and avoid assertion-library edge cases, assert with double.IsNaN(...).

        Math.Log2(-1.0).Should().Be(double.NaN);
        Math.Log2(double.NaN).Should().Be(double.NaN);

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[Tyrrrz]

@copilot this method was added only in .NET Core 3.0. It's not available in .NET Standard.

[Copilot]

@copilot this method was added only in .NET Core 3.0. It's not available in .NET Standard.

Fixed in commit Fix #if guard for Math.Log2: applies to all NETSTANDARD targets. Changed the guard from (NETSTANDARD && !NETSTANDARD2_1_OR_GREATER) to (NETSTANDARD) so the polyfill now applies to all .NET Standard targets (1.x, 2.0, and 2.1), since Math.Log2 is not available in any .NET Standard version.