refactor(pr-x1): move SIMD primitives to simd_soa.rs

Per the layering rule: SIMD substrate primitives live at the crate
root in `simd_{type}.rs` files, dispatched through `simd.rs > crate::simd`.
`src/hpc/column.rs` and `src/hpc/array_chunks.rs` violated that — moved
to `src/simd_soa.rs`.

  src/hpc/column.rs       → src/simd_soa.rs (MultiLaneColumn)
  src/hpc/array_chunks.rs → src/simd_soa.rs (array_chunks + array_chunks_checked)

`src/simd.rs` now does `pub use crate::simd_soa::{…}` — the W1a contract
path is `use ndarray::simd::*`, consumers never reach into `simd_soa`
directly.

`src/lib.rs` adds `pub mod simd_soa;` alongside `simd_avx512`, `simd_neon`,
`simd_amx`, etc. — same `#[cfg(feature = "std")]` gating as siblings.

`src/hpc/mod.rs` drops the two `pub mod` declarations; the doc-comment
now records why these are NOT in `hpc::*`.

All doctests updated to the canonical `use ndarray::simd::*;` path.

Author: claude

src/hpc/array_chunks.rs

@@ -270,6 +270,14 @@ pub mod simd_neon;
 #[allow(clippy::all, missing_docs, dead_code, unused_variables, unused_imports)]
 pub mod simd_wasm;
 
+// PR-X1 — SoA-shaped SIMD substrate primitives (`MultiLaneColumn`,
+// `array_chunks`). Layout-only; the SIMD register load happens in the
+// consumer's loop using `crate::simd::F32x16::from_array` etc. Re-exported
+// through `crate::simd::*` per the W1a consumer contract.
+#[cfg(feature = "std")]
+#[allow(clippy::all, missing_docs, dead_code, unused_variables, unused_imports)]
+pub mod simd_soa;
+
 /// Slice-level integer SIMD ops (i8/i16) — `add_i8`, `dot_i8`, `min_i8`, …
 #[cfg(feature = "std")]
 #[allow(missing_docs)]