refactor(substrate): graduate 5 modules from hpc/ to crate root

Continues the substrate-graduation thread documented in #192's wrap-up
and extended in #193's simd_caps lift. Five more modules move from
`crate::hpc::*` (the rustynum migration staging area) to crate root
where they sit alongside `simd.rs`, `simd_runtime/`, `simd_caps`, and
the W1a polyfill surface they're supposed to compose with.

| Module | Reason |
|---|---|
| `bitwise`        | Pure SIMD primitives (popcount, hamming over byte slices); already uses `crate::simd::U64x8` polyfill internally; already re-exported via `simd.rs:512`. |
| `heel_f64x8`     | All-F64x8 polyfill consumer (dot, cosine, sum-sq, weighted-hamming); already re-exported via `simd.rs:563`. |
| `distance`       | Spatial 3D + slice-shape L1/L2/L∞ (PR-X10 A6); the linalg/mod.rs hard-boundary comment now points here at root. |
| `byte_scan`      | Pure SIMD utility (needle search, delimiter find). |
| `spatial_hash`   | Pure SIMD utility (bucketing, candidate gather). |

# Why these five, why now

All five satisfied the low-hanging-fruit criteria from #193's wrap-up
discussion:
  1. No internal `hpc/` dependencies (only `super::simd_caps` which
     still resolves correctly because `simd_caps` is itself at crate
     root post-#192).
  2. Already polyfill-clean — no raw-intrinsic refactor needed before
     the move.
  3. Already partially exposed via `crate::simd::*` re-exports.

The next graduation tier (`fingerprint`, `dn_tree`, `ogit_bridge`,
`splat3d`) needs a polyfill audit before it can move, and
`fingerprint` in particular is gated on the W1a-#5 POPCOUNT-U64
primitive landing (so its bit ops can route through `U64xN.popcnt()`
instead of raw `u64.count_ones()`).

# Back-compat preserved end-to-end

Every cross-repo consumer using `ndarray::hpc::{bitwise, heel_f64x8,
distance, byte_scan, spatial_hash}::*` continues to compile
unmodified. The `src/hpc/mod.rs` declarations change from
`pub mod X;` to `pub use crate::X;` — Rust re-exports modules just
like other items, so `crate::hpc::X::*` resolves through to the same
items as `crate::X::*`. Internal `super::simd_caps::simd_caps()`
calls inside the moved files continue to work because `super::` at
crate root resolves to `crate::*` which has `simd_caps` (graduated
in #192).

# Changes

- `git mv` five files from `src/hpc/` to `src/`.
- `src/lib.rs` gains five `#[cfg(feature = "std")] pub mod X;`
  declarations next to the existing `simd_caps` block, each with a
  one-liner docstring naming the graduation source and the
  substrate-tier reason for the move.
- `src/hpc/mod.rs` replaces five `pub mod X;` with `pub use crate::X;`
  (back-compat re-exports).
- `src/hpc/linalg/mod.rs` updates the hard-boundary comment from
  "No distance metrics — those live in `crate::hpc::distance`" to
  point at `crate::distance` (the new canonical path) with a
  parenthetical noting the back-compat re-export.
- The `bitwise.rs` declaration in `src/hpc/mod.rs` is now a comment
  instead of being interleaved with `pub mod hdc`/`pub mod projection`
  to make the graduation status visible at a glance.

# Verification

- `cargo build -p ndarray --lib` — clean
- `cargo build -p ndarray --lib --no-default-features` — clean
  (the new `#[cfg(feature = "std")]` gates match the existing
  `simd_caps` pattern; nostd targets see no change)
- `cargo test -p ndarray --lib bitwise:: distance:: heel_f64x8::
  byte_scan:: spatial_hash::` — all 119 tests on the five graduated
  modules pass at the new path (test names now `bitwise::tests::*`
  rather than `hpc::bitwise::tests::*`)
- `cargo test -p ndarray --lib --features "pillar,ogit_bridge,
  runtime-dispatch" hpc::` — 2167 passed, 0 failed, 28 ignored
- `cargo fmt --all --check` — clean
- `cargo clippy --features "pillar,ogit_bridge,runtime-dispatch"
  --lib -- -D warnings` — clean

# Next graduation candidates (deferred)

- `hpc::fingerprint` — needs W1a-#5 POPCOUNT-U64 to land first so
  bit ops can route through `U64xN.popcnt()` instead of raw
  `u64.count_ones()`. Cognitive-shader-foundation explicitly names
  `Fingerprint<N>` as a MUST-be-in-`ndarray::simd::*` type.
- `hpc::dn_tree` (bitwise core) — same polyfill-audit dependency.
  The cognitive DNTree/DNConfig/TraversalHit state stays in
  `hpc/` after the split.
- `hpc::ogit_bridge` — pure logic, no SIMD, can move once the
  fingerprint + dn_tree audits are out of the way (avoids three
  partial graduations in flight at once).
- `hpc::splat3d` — already mostly polyfill-clean; pure path
  rewrite. Defer because it's a larger consumer surface than
  the five in this PR.

Author: claude

src/hpc/bitwise.rs src/bitwise.rssrc/hpc/bitwise.rs renamed to src/bitwise.rs

@@ -40,7 +40,8 @@
 //!
 //! - **No SIMD primitives** — use `crate::simd::{F32x16, …}` directly.
 //! - **No `#[target_feature]` annotations** — those live in `simd_avx512.rs`.
-//! - **No distance metrics** — those live in `crate::hpc::distance`.
+//! - **No distance metrics** — those live in `crate::distance` (graduated
+//!   from `crate::hpc::distance`; back-compat re-export in `crate::hpc::*`).
 
 mod matrix;
 pub use matrix::{Mat2, Mat3, Mat4, MatN, Spd2, Spd3};

src/hpc/byte_scan.rs src/byte_scan.rssrc/hpc/byte_scan.rs renamed to src/byte_scan.rs

@@ -27,7 +27,8 @@ pub mod reductions;
 pub mod statistics;
 pub mod activations;
 pub mod hdc;
-pub mod bitwise;
+// Bitwise SIMD primitives — graduated to crate root. Back-compat re-export.
+pub use crate::bitwise;
 pub mod projection;
 pub mod cogrecord;
 pub mod graph;
@@ -56,8 +57,8 @@ pub mod soa;
 pub mod node;
 #[allow(missing_docs)]
 pub mod cascade;
-#[allow(missing_docs)]
-pub mod heel_f64x8;
+// HEEL F64x8 distance kernels — graduated to crate root. Back-compat re-export.
+pub use crate::heel_f64x8;
 // AMX is an x86_64-only ISA (Intel Sapphire Rapids+); both modules use
 // `asm!` with `rcx`/`rax` register names that don't exist on other
 // architectures (rejected at parse time on s390x / aarch64 / wasm32).
@@ -169,10 +170,11 @@ pub mod parallel_search;
 // ZeckF64 progressive edge encoding + batch/top-k
 pub mod zeck;
 
-// SIMD-accelerated spatial / byte-scan / hash utilities
-pub mod distance;
-pub mod byte_scan;
-pub mod spatial_hash;
+// SIMD-accelerated spatial / byte-scan / hash utilities — graduated to crate root.
+// Back-compat re-exports for existing `use ndarray::hpc::{distance,byte_scan,spatial_hash}::*`.
+pub use crate::byte_scan;
+pub use crate::distance;
+pub use crate::spatial_hash;
 
 // Variable-width palette index codec (Minecraft-style bit packing)
 #[allow(missing_docs)]

src/hpc/distance.rs src/distance.rssrc/hpc/distance.rs renamed to src/distance.rs

@@ -272,6 +272,35 @@ pub mod simd_amx;
 #[cfg(feature = "std")]
 pub mod simd_caps;
 
+/// Bitwise SIMD primitives — popcount, Hamming distance over byte slices.
+/// Graduated from `crate::hpc::bitwise::*` (substrate-tier; uses
+/// `crate::simd::U64x8` polyfill internally). Back-compat re-export in
+/// `crate::hpc::*` preserves existing import paths.
+#[cfg(feature = "std")]
+pub mod bitwise;
+
+/// F64x8 HEEL distance kernels — 8-plane weighted Hamming, f64 SIMD
+/// dot / cosine / sum-of-squares. Graduated from `crate::hpc::heel_f64x8::*`.
+#[cfg(feature = "std")]
+pub mod heel_f64x8;
+
+/// Batch distance computations — spatial 3D-point queries +
+/// slice-shape L1 / L2 / L∞ (PR-X10 A6). Graduated from
+/// `crate::hpc::distance::*`.
+#[cfg(feature = "std")]
+pub mod distance;
+
+/// SIMD-accelerated byte-scan utilities — needle search, delimiter
+/// finding, parallel byte comparison. Graduated from
+/// `crate::hpc::byte_scan::*`.
+#[cfg(feature = "std")]
+pub mod byte_scan;
+
+/// SIMD-accelerated spatial hash — bucketing, candidate gather, hash
+/// collision detection. Graduated from `crate::hpc::spatial_hash::*`.
+#[cfg(feature = "std")]
+pub mod spatial_hash;
+
 #[cfg(feature = "std")]
 #[allow(clippy::all, missing_docs, dead_code, unused_variables, unused_imports)]
 pub mod simd_neon;