fix(ci): allow dead_code on Tier enums for non-x86_64 cross-builds

i686-unknown-linux-gnu cross-tests (which run under -D warnings) failed:

  error: variants `Avx512` and `Avx2` are never constructed
    --> src/backend/native.rs:17:5
       = note: `-D dead-code` implied by `-D warnings`

Two `enum Tier { Avx512, Avx2, ..., Scalar }` definitions exist, one in
`src/backend/native.rs` and one in `src/simd.rs`. Both `detect_tier()`
blocks gate the AVX-detection paths behind `#[cfg(target_arch =
"x86_64")]`; the NEON-detection (simd.rs only) is gated behind
`#[cfg(target_arch = "aarch64")]`. On i686 / wasm32 / etc., none of
those gates match → only `Scalar` is ever constructed → dead-code
lint fires on the other variants.

Fix: `#[allow(dead_code)]` on both Tier enums. The variants ARE needed
on x86_64 and aarch64 builds (the cfg blocks produce them via runtime
feature detection); they just don't reach instantiation under the i686
target_arch.

The src/simd.rs fix is preemptive — only src/backend/native.rs was
flagged in the CI error, but src/simd.rs has 4 non-Scalar variants
(Avx512, Avx2, NeonDotProd, Neon) that would trigger the same lint
under -D warnings on i686. Both fixed in one commit.

Verification:
- `cargo clippy --no-deps` (default x86_64) → clean (the variants ARE
  constructed via `is_x86_feature_detected!("avx512f")`)
- `RUSTFLAGS="-D dead_code" cargo check --lib --features ...` → clean

Author: claude

src/backend/native.rs

@@ -12,6 +12,12 @@ use std::sync::LazyLock;
 
 // ─── Tier detection: happens ONCE, at first access ─────────────────
 
+// On non-x86_64 targets (e.g. i686 / aarch64 / s390x cross-tests) only the
+// `Scalar` variant is ever constructed — the AVX detection block below is
+// gated `#[cfg(target_arch = "x86_64")]`. Without `dead_code` allowance the
+// `-D warnings` build fails on i686 with `variants Avx512 and Avx2 are
+// never constructed`.
+#[allow(dead_code)]
 #[derive(Clone, Copy, PartialEq)]
 enum Tier {
     Avx512,

src/simd.rs

@@ -8,6 +8,11 @@
 #[cfg(feature = "std")]
 use std::sync::LazyLock;
 
+// On i686 / wasm32 / etc. only the `Scalar` variant is constructed —
+// `detect_tier()`'s feature-detection blocks are `target_arch = "x86_64"`
+// or `"aarch64"` gated, both false on i686. Without `dead_code` allowance
+// the `-D warnings` build fails with `variants ... are never constructed`.
+#[allow(dead_code)]
 #[derive(Clone, Copy, PartialEq, Debug)]
 #[repr(u8)]
 enum Tier {