[AdaWorldAPI]

Summary

#372 merged at its first commit (e07be5b). That commit's narrow is off the 512-bit ladder: I32x16 -> I8x16/U8x16, a 128-bit result. The ladder rework reached the #372 branch after the merge (dc26129), so it never landed. This PR applies that same diff to master: it removes the I8x16/U8x16 narrow and adds the ladder steps on every backend.

• I32x16::narrow_saturating_i16(hi) -> I16x32
• I16x32::narrow_saturating_i8(hi) -> I8x64
• I16x32::narrow_saturating_u8(hi) -> U8x64

Lanes 0..N come from self and N..2N from hi. Each lane is clamped to the narrow range.

backend	how
AVX-512	vpmovsdw / vpmovswb / vpmovuswb, then vinserti64x4 (u8 first raises negative lanes to 0)
AVX2 / AVX	SSE2 packssdw / packsswb / packuswb over 16-byte quarters
NEON	sqxtn / sqxtun + vcombine
wasm simd128	i16x8_narrow_i32x4, i8x16_narrow_i16x8, u8x16_narrow_i16x8
scalar	clamp
nightly	simd_clamp + cast

Two backend notes:

• wasm: I16x32 / I8x64 / U8x64 come from the scalar backend on wasm. Their narrows are therefore implemented in simd_wasm.rs, and the scalar versions are cfg-gated off for that arm.
• aarch64: the facade now re-exports the NEON I16x32 / I8x64 (+ i16x32 / i8x64). simd_neon.rs already had both types, but no arm re-exported them.

Nightly keeps F32x16::cast_i32 and I32x16::cast_f32.

Verification

git diff origin/master on this branch equals git diff e07be5b dc26129 (checked with diff), so the measurements taken on dc26129 apply unchanged:

• Parity group 0x10xx in simd-masking-parity:
  • inputs: an edge set plus 63 random rounds;
  • the chained i32→i16→i8/u8 result is compared with a single clamp;
  • the in-range f32↔i32 casts are compared with as;
  • anti-vacuity: some lanes must saturate and some must pass through unchanged.
• All 8 arms pass, 17 groups bit-identical: native v4, native v3, avx-qemu, v2-qemu, neon-qemu, wasm, wasm-scalar, nightly.
• Disable run: with the AVX-512 u8 "raise negatives to 0" step removed, the parity check fails with 0x1080, the u8 lane holding i32::MIN.
• Lowered assembly (#[inline(never)] wrappers, release):
  • v4: two vshufi64x2 + one vpackssdw / vpacksswb / vpackuswb per narrow;
  • v3: four 128-bit packs per narrow;
  • no scalar code on either.
• cargo clippy --lib -D warnings is clean on v4 and v3. The new doctests pass (v4, release).

---

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-24T17:41:56.445767Z	9dc5862	Draft marked ready

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