Document InlineArrayAttribute as the correct approach for fixed-size buffers

[Copilot]

Users attempting to use StructLayoutAttribute.Size to create fixed-size buffers encounter memory corruption because the Size property only affects unmanaged layout, not managed memory allocation for non-blittable types.

Changes

• customize-struct-marshalling.md: Added "Fixed-size buffers and inline arrays" section

  • Warns against using StructLayoutAttribute.Size for fixed-size buffers
  • Explains technical limitation: only affects managed layout for blittable types
  • Provides dangerous vs. safe example using InlineArrayAttribute
  • Cross-references C# language documentation

• apply-interop-attributes.md: Added cross-reference to InlineArrayAttribute in the StructLayoutAttribute table entry

Example

// ❌ Dangerous: only allocates 1 byte, corrupts memory beyond
[StructLayout(LayoutKind.Explicit, Size = 100)]
struct UnsafeBuffer
{
    [FieldOffset(0)]
    private byte b;
}

// ✔️ Safe: allocates full 100 bytes
struct SafeBuffer
{
    private Buffer100 buffer;
}

[InlineArray(100)]
internal struct Buffer100
{
    private byte _element0;
}

Original prompt

Please implement the suggestion from the user in the last common on this page: dotnet/runtime#112104

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