Replace Marshal.SizeOf with sizeof operator

[vcsjones]

This implements the suggestion from @jkotas to use sizeof instead of Marshal.SizeOf for blittable structs, or to stop using the native heap entirely for some others.

#120342 (comment)

This does not do the one in StorePal.Windows.Export.cs‎ since I assumed that one will be taken in the linked PR.

[Copilot]

Pull Request Overview

Replaces Marshal.SizeOf() calls with the sizeof operator for blittable structs and removes unnecessary native heap allocations in favor of stack usage to reduce overhead and improve performance.

• Converted Marshal.SizeOf usages to sizeof where applicable.
• Simplified unsafe interop structures by using stack locals instead of AllocHGlobal + StructureToPtr.
• Narrowed marshalling overhead in ECCng named curve import path.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
ChainPal.Windows.BuildChain.cs	Replaces Marshal.SizeOf with sizeof and introduces an unsafe block for struct size retrieval.
PqcBlobHelpers.cs	Marks method unsafe and uses sizeof instead of Marshal.SizeOf; removes redundant inner unsafe block.
MLKem.Windows.cs	Replaces Marshal.SizeOf with sizeof for key blob header sizing.
ECCng.ImportExport.NamedCurve.cs	Eliminates unmanaged allocations and marshaling by using stack-allocated structs and passing their addresses directly.

[jkotas]

Thanks