fix: make simd module public (was pub(crate))

Consumers (bgz-tensor, thinking-engine) need ndarray::simd::bf16_to_f32_batch
and other polyfill exports. Was incorrectly restricted to pub(crate).

https://claude.ai/code/session_01ChLvBfpJS8dQhHxRD4pYNp

Author: claude

src/lib.rs

@@ -232,7 +232,7 @@ mod dimension;
 /// Portable SIMD types — `crate::simd::f32x16` today, `std::simd::f32x16` tomorrow.
 #[cfg(feature = "std")]
 #[allow(missing_docs)]
-pub(crate) mod simd;
+pub mod simd;
 #[cfg(all(feature = "std", target_arch = "x86_64"))]
 #[allow(missing_docs, dead_code)]
 pub(crate) mod simd_avx512;