…t re-export

First step in the substrate-graduation thread documented in #192's
wrap-up: lift the substrate-tier modules out of `hpc/` (which was
the rustynum migration staging area) to crate root, where they sit
in scope of the W1a polyfill contract and no longer carry the
spurious `std`-gate inherited from `hpc/`.

`simd_caps` is the smallest and cleanest first move:
  * No internal `hpc/` dependencies (only `use std::sync::LazyLock`).
  * 8 internal callers; back-compat re-export keeps them working.
  * Pure CPU-detection metadata; the most polyfill-adjacent module
    in the entire `hpc/` set.

Changes:

1. `src/hpc/simd_caps.rs` → `src/simd_caps.rs` (file move).
2. `src/lib.rs` adds `#[cfg(feature = "std")] pub mod simd_caps;`.
   The std-gate is retained for now (uses `std::sync::LazyLock`);
   lifting it to `core::sync::LazyLock` is a separate follow-up.
3. `src/hpc/mod.rs` replaces `pub mod simd_caps;` with
   `pub use crate::simd_caps;` — keeps `crate::hpc::simd_caps::*`
   resolving for cross-repo consumers (lance-graph, WoA, MedCare,
   q2 may have `use ndarray::hpc::simd_caps::*` imports that this
   preserves untouched).

No public-API breakage; the test suite picks up the new path
(test names now `simd_caps::tests::*` rather than `hpc::simd_caps::*`),
all 10 tests pass under both default and `runtime-dispatch` configs.
The 8 internal callers (`crate::simd_avx512`, `crate::hpc::p64_bridge`,
`crate::simd_runtime::{cpu_ops, add_mul, vnni_dot}`) continue using
`crate::hpc::simd_caps::*` via the re-export and work unmodified.

Next graduation candidates (deferred to follow-up PRs):
  - `fingerprint` (bitwise substrate; raw `u64` polyfill audit)
  - `dn_tree` (bitwise substrate; same audit)
  - `ogit_bridge` (pure logic, no SIMD primitives)
  - `splat3d` (already uses `crate::simd::*` polyfilled types)

Each move follows the same pattern: relocate file, drop std-gate
inheritance where unneeded, keep back-compat re-export. Cognitive
layer (pillar, plane, seal, merkle_tree, deepnsm, …) stays inside
`hpc/` and keeps its legitimate std-gate.

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.

Continues the substrate-graduation thread from #192 (simd_caps),
#193 (clippy/doc cleanup), and #194 (bitwise/heel_f64x8/distance/
byte_scan/spatial_hash). Same low-hanging-fruit criteria — no
internal hpc/ deps, polyfill-clean, single-line back-compat shim
keeps every existing import resolving.

| Module          | Reason                                                         |
|---|---|
| `aabb`          | SIMD AABB intersection/expansion/distance; only deps are       |
|                 | `crate::simd::F32x16` + `super::simd_caps` (graduated #192).   |
| `nibble`        | 4-bit packed nibble batch ops; only dep is `crate::simd::U8x64`.|
| `palette_codec` | Variable-width palette index codec (1-8 bit packing); zero deps.|
| `property_mask` | AVX-512 VPTERNLOGD bitset queries on block state bits;         |
|                 | only dep is `crate::simd::U64x8`.                              |

# Why these four, why now

All four satisfy the criteria from #194's wrap-up:
  1. No internal `hpc/` dependencies — only `crate::simd::*`
     (polyfill surface) and `super::simd_caps` (which is itself
     at crate root post-#192).
  2. Polyfill-clean — no raw-intrinsic refactor required.
  3. Single in-tree downstream caller (`hpc::framebuffer` uses
     `palette_codec`) → the `pub use crate::palette_codec;`
     back-compat shim keeps that resolution working zero-touch.

# Mechanical changes

- `git mv src/hpc/{aabb,nibble,palette_codec,property_mask}.rs src/`
- `src/lib.rs`: added four `pub mod` declarations under
  `#[cfg(feature = "std")]`, each with a `# Example` rustdoc block
  per CLAUDE.md "all public APIs need doc comments with examples".
- `src/hpc/mod.rs`: replaced the four `pub mod` declarations with
  `pub use crate::{aabb, nibble, palette_codec, property_mask};`
  back-compat re-exports. `crate::hpc::aabb::*` and friends keep
  resolving for every existing call site, identical to how
  `crate::hpc::bitwise::*` works post-#194.

# Clippy / lint cleanup

17 clippy errors surfaced under `-D warnings` once the modules
left the `hpc/mod.rs` `#![allow(clippy::all, ...)]` umbrella.
Fixed each at the canonical Rust idiom (the #194 cleanup pattern,
417131b), no umbrella re-application:

- **manual_div_ceil (6 sites)** — `(n + d - 1) / d` → `n.div_ceil(d)`
  in `nibble.rs` (x2), `palette_codec.rs` (x3), `property_mask.rs`.
- **needless_range_loop (10 sites)** — `for i in start..vec.len()`
  rewrites to `for x in &vec[start..]` (when index unused) or
  `for (i, &x) in iter().enumerate().skip(start)` (when index used).
  Sites: `aabb.rs` x4, `nibble.rs` x3, `palette_codec.rs` x1,
  `property_mask.rs` x2.
- **missing_docs (4 sites)** — added field doc comments on
  `pub struct Aabb { min, max }` and `pub struct Ray { origin,
  inv_dir }`. Previously masked by the `hpc/mod.rs` umbrella's
  `#![allow(missing_docs)]`.

# Doctest correction

Initial `# Example` in `src/lib.rs` for `palette_codec` asserted
`bits_for_palette_size(1) == 1` per the module's own docstring
table, but the impl returns 0 for `palette_size <= 1` (trivial-
palette special case). Changed assertion to use `bits_for_palette_
size(2) == 1` — exercises the same code path with input the impl
actually handles per spec.

# Verification

```
cargo check --lib                                          green
cargo clippy --lib -- -D warnings                          green
cargo clippy --lib --features rayon -- -D warnings         green
cargo clippy --features approx,serde,rayon -- -D warnings  green
cargo test --doc (15 graduated-module doctests)            pass
cargo test --lib (104 unit tests across 4 modules)         pass
```

# What's next

`hpc/` inventory: ~55 → ~51 modules at the staging path. Next-batch
candidates per the same criteria need a deps audit before move:
`framebuffer` (uses `palette_codec` shim, otherwise crate-root),
`ocr_simd`/`ocr_felt`, `audio`. Filed in AGENT_LOG entry for the
follow-up pass.

https://claude.ai/code/session_01HbqooFZHAjaUtFEzhA1R2u