Implement various convenience methods for System.Numerics types

[tannergooding]

This resolves #103464

[Copilot]

Pull Request Overview

This PR exposes several convenience methods for System.Numerics types by promoting internal conversion and transformation routines to public APIs. Key changes include making methods such as AsPlane, AsQuaternion, CreateScalar, and various Transform overloads public, along with adding new extension methods and updating related tests.

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Runtime.Intrinsics/ref/System.Runtime.Intrinsics.cs	Added new public conversion methods (AsPlane, AsQuaternion) on Vector128.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector128.Numerics.cs	Updated accessibility of conversion methods from internal to public.
src/libraries/System.Private.CoreLib/src/System/Numerics/*	Exposed convenience methods (CreateScalar, Transform, etc.) for Vector2, Vector3, Vector4, Plane, Quaternion and updated overloads accordingly.
src/libraries/System.Numerics.Vectors/tests/*	Added tests to cover the new public API methods.
src/libraries/System.Numerics.Vectors/ref/System.Numerics.Vectors.cs	Updated public API surface reflecting the new methods and indexers.

Comments suppressed due to low confidence (2)

src/libraries/System.Numerics.Vectors/ref/System.Numerics.Vectors.cs:26

• Ensure that the newly exposed public indexers and convenience methods are accompanied by complete and updated documentation in the official API docs. This will help consumers understand the intended usage of the new methods.

public System.Numerics.Vector2 this[int row, int column] { readonly get { throw null; } set { throw null; } }

src/libraries/System.Private.CoreLib/src/System/Numerics/Vector4.cs:970

• Passing Matrix4x4 by value instead of using an 'in' parameter may introduce additional copying overhead compared to the previous pattern. Please confirm that this design change has been benchmarked and is acceptable regarding performance.

public static Vector4 Transform(Vector2 position, Matrix4x4 matrix)

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