SIMD and vectorization in dotnet/runtime

The general, cross-cutting guidance for SIMD and hardware intrinsics lives in the official .NET documentation. Read it first and defer to it for anything not specific to this repo:

• Use SIMD and hardware intrinsics in .NET

The repo-specific nuance is in docs/coding-guidelines/vectorization-guidelines.md. This skill distills what to actually enforce when authoring or reviewing vectorized changes here.

Core rules