Refactor the various System.Numerics Vector/Matrix/Quaternion/Plane types to require less inlining

[tannergooding]

The existing code was heavily reliant on often 3-4 layers of inlining per call and was inlining some larger functions which didn't need to be inlined while missing some smaller functions that were just above the "always inline" threshold.

This refactors the code to be less reliant on the inliner, by ensuring the core functions are doing direct handling where applicable.

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[Copilot]

Pull request overview

This PR refactors several System.Numerics vector/matrix/quaternion/plane implementations to rely less on multi-layer wrapper inlining by routing more operations directly through Vector128<T> helpers and by moving some previously “Impl-file” logic inline.

Changes:

• Adds Vector128<float>-level helpers (e.g., Distance, Length, Normalize) and updates Vector2/3/4 APIs to call them directly.
• Refactors Vector2/3/4, Quaternion, and Plane operations to use Vector128<T> plumbing more consistently (and adds/adjusts inlining attributes).
• Removes Matrix3x2.Impl.cs / Matrix4x4.Impl.cs from compilation inputs, moving needed implementation into the primary source files.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector128.Numerics.cs	Adds internal Vector128<float> Numerics helpers (distance/length/normalize).
src/libraries/System.Private.CoreLib/src/System/Numerics/Vector4.cs	Refactors many ops to Vector128; changes span create/copy and ToString implementation.
src/libraries/System.Private.CoreLib/src/System/Numerics/Vector3.Extensions.cs	Adjusts Vector3 pointer store and AsVector2 path; simplifies unaligned stores.
src/libraries/System.Private.CoreLib/src/System/Numerics/Vector3.cs	Refactors ops to Vector128; adds internal helpers; updates ToString implementation.
src/libraries/System.Private.CoreLib/src/System/Numerics/Vector2.Extensions.cs	Adjusts Vector2 pointer store paths and unaligned stores.
src/libraries/System.Private.CoreLib/src/System/Numerics/Vector2.cs	Refactors ops to Vector128; adds internal transform helpers; updates ToString implementation.
src/libraries/System.Private.CoreLib/src/System/Numerics/Quaternion.cs	Refactors quaternion ops to Vector128 and adds internal helpers/overloads.
src/libraries/System.Private.CoreLib/src/System/Numerics/Plane.cs	Refactors plane ops to Vector128 and updates transform/normalize logic.
src/libraries/System.Private.CoreLib/src/System/Numerics/Matrix4x4.Impl.cs	Removes the standalone Impl file (implementation expected elsewhere).
src/libraries/System.Private.CoreLib/src/System/Numerics/Matrix3x2.Impl.cs	Removes the standalone Impl file (implementation moved into Matrix3x2.cs).
src/libraries/System.Private.CoreLib/src/System/Numerics/Matrix3x2.cs	Moves/rewrites Impl logic inline; refactors operators and helpers.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Stops compiling removed Impl files; minor formatting normalization.

Copilot's findings

• Files reviewed: 12/13 changed files
• Comments generated: 7

[MichalPetryka]

@MihuBot -nuget

[Copilot]

Copilot's findings

• Files reviewed: 12/13 changed files
• Comments generated: 12

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Private.CoreLib/src/System/Numerics/Quaternion.cs:430

• Quaternion.Slerp changed the sign check from cosOmega < 0.0f to float.IsNegative(cosOmega). Those are not equivalent: IsNegative treats -0 as negative and can treat NaN with the sign bit set as negative, which changes the branch behavior and can flip the sign of the returned quaternion components. If the intent is to preserve previous comparison semantics, use the original < 0.0f check.

 float sign = 1.0f;
if (float.IsNegative(cosOmega))
{
cosOmega = -cosOmega;
sign = -1.0f;
}

• Files reviewed: 12/13 changed files
• Comments generated: 3

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Private.CoreLib/src/System/Numerics/Quaternion.cs:430

• Quaternion.Slerp changed from cosOmega < 0.0f to float.IsNegative(cosOmega), which changes the branching behavior for -0.0 (and for some NaN sign-bit cases). If this semantic change is intentional, it should be covered by a targeted regression test (e.g., construct quaternions that yield cosOmega == -0.0f and verify the expected interpolation result) so the behavior is locked in.

 float cosOmega = Dot(quaternion1, quaternion2);
float sign = 1.0f;
if (float.IsNegative(cosOmega))
{
cosOmega = -cosOmega;
sign = -1.0f;
}

• Files reviewed: 12/13 changed files
• Comments generated: 1