feat(hpc/soa): SoaVec + soa_struct! + aos_to_soa + soa_to_aos (W3+W5+W6)

New module src/hpc/soa.rs implementing the SoA/AoS layout helpers per
.claude/knowledge/w3-w6-soa-aos-design.md v2 (commit 10151d7).

W3: SoaVec<T,N> generic container ([Vec<T>; N] inside) with runtime
field(i) + compile-time field_n::<I>(), chunks(k), all_fields().
soa_struct! macro generates named-field structs with Vec<T> per field
and inherent new/push/len/clear/Default.

W5+W6: aos_to_soa<T,N,F> scalar deinterleave via closure;
soa_to_aos<T,N,F> scalar interleave inverse. Co-located with SoaVec
per plan-review P0-1 (NOT in simd_ops.rs — that module is SIMD-only).

Scalar implementations throughout. No #[target_feature], no
simd_{type}.rs imports, no cfg(target_feature). Forward-compatible
with future bench-justified SIMD swap via grow-internal-arms.

Out of scope: distance metrics
(see .claude/knowledge/cognitive-distance-typing.md).

Author: claude

src/hpc/mod.rs

@@ -43,6 +43,8 @@ pub mod plane;
 #[allow(missing_docs)]
 pub mod seal;
 #[allow(missing_docs)]
+pub mod soa;
+#[allow(missing_docs)]
 pub mod node;
 #[allow(missing_docs)]
 pub mod cascade;