Add polyfills for Math.Clamp(...)

[Tyrrrz]

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[Copilot]

Pull Request Overview

This PR adds polyfills for Math.Clamp(...) to enable this API on older .NET framework versions.

• Adds generic Math.Clamp<T> polyfill for frameworks before .NET Core 2.0
• Adds Math.Clamp for IntPtr (nint) for frameworks before .NET 6.0
• Includes test coverage for both polyfill implementations

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
PolyShim/NetCore20/Math.cs	Implements generic Math.Clamp<T> polyfill using IComparable<T> for frameworks lacking .NET Core 2.0 APIs
PolyShim/Net60/Math.cs	Implements Math.Clamp for IntPtr/nint type for frameworks lacking .NET 6.0 APIs
PolyShim.Tests/NetCore20/MathTests.cs	Adds tests for generic clamp implementation covering integer and double types
PolyShim.Tests/Net60/MathTests.cs	Adds tests for IntPtr clamp implementation covering basic clamping scenarios

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