feat: make simd_avx2 module public for ladybug-rs dot_i8 access

ladybug-rs needs ndarray::simd_avx2::dot_i8() for VNNI int8 dot product.
Changed from pub(crate) to pub.

https://claude.ai/code/session_01CdqyUTUfjKZuk8YGJzv6LB

Author: claude

src/lib.rs

@@ -238,7 +238,7 @@ pub(crate) mod simd;
 pub(crate) mod simd_avx512;
 #[cfg(all(feature = "std", target_arch = "x86_64"))]
 #[allow(missing_docs)]
-pub(crate) mod simd_avx2;
+pub mod simd_avx2;
 
 /// Pluggable linear algebra backends (native SIMD, MKL, OpenBLAS).
 #[cfg(feature = "std")]