refactor(simd_caps): graduate from hpc/ to crate root with back-compat 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.

Author: claude

src/hpc/mod.rs

@@ -13,8 +13,10 @@
 //! - FFT (forward, inverse, real-to-complex)
 //! - VML (vectorized math library)
 
-// SIMD capability singleton — detect once, all modules share
-pub mod simd_caps;
+// SIMD capability singleton — graduated to crate root (it never depended
+// on anything else in `hpc/`); re-exported here for back-compat with
+// existing `crate::hpc::simd_caps::*` imports across the workspace.
+pub use crate::simd_caps;
 // LazyLock frozen SIMD dispatch — function pointers selected once at startup
 pub mod simd_dispatch;
 

src/lib.rs

@@ -262,6 +262,16 @@ pub mod simd_nightly;
 #[cfg(target_arch = "x86_64")]
 pub mod simd_amx;
 
+/// SIMD capability detection (CPUID on x86_64, runtime feature detection
+/// on aarch64). One `LazyLock<SimdCaps>` detected at first access; every
+/// substrate dispatch site is one pointer deref. Graduated from
+/// `crate::hpc::simd_caps::*` in this same migration; the old path stays
+/// available as a `pub use` re-export inside `crate::hpc::*` for
+/// back-compat. Uses `std::sync::LazyLock`, hence the `std` gate (a
+/// `core::sync::LazyLock` follow-up could lift it).
+#[cfg(feature = "std")]
+pub mod simd_caps;
+
 #[cfg(feature = "std")]
 #[allow(clippy::all, missing_docs, dead_code, unused_variables, unused_imports)]
 pub mod simd_neon;