Fix the merge regression that broke every compile job

My 2026-08-11 merge of origin/master kept BOTH sides of the simd.rs
re-export block, which introduced a duplicate `bf16_tile_gemm_16x16`
name: once unaliased from `hpc::bf16_tile_gemm` (this branch's W1a
re-export) and once from `simd_ops` (master's canonical polyfill
export). The collision is a hard compile error, which is why clippy,
tests, native-backend, tier4-avx512, wasm-simd, neon-qemu and
hpc-stream-parallel all went red at once on 921bd7a.

Resolution: drop this branch's unaliased re-export and keep master's
deliberate two-name split — `bf16_tile_gemm_16x16` (the pure polyfill
kernel, from simd_ops) and `bf16_tile_gemm_16x16_amx` (the
tile-dispatching wrapper, from hpc). Master's arrangement already
satisfies the W1a "reach it via ndarray::simd::*" intent that this
branch's export was written for, so nothing is lost. No tile-GEMM
function is added or removed by this branch — `int8_tile_gemm_16x16`
(used by the geostrophic example) has been on master since 58200fb,
and no 16x32 tile GEMM exists in the crate; the only 16x32 symbols are
SIMD register types (I16x32/U16x32) in simd_nightly, which are lane
widths, not tile kernels. 16x16 is AMX's native TDPBUSD accumulator
geometry, not a choice.

Also in this commit:
- Restore the newline my merge script ate between the `_amx` re-export
  and the CPU-generation comment (this was the `format/stable` failure).
- Correct the arch_prctl wording in AMX_GOTCHAS Gotcha 15: 158 is the
  x86_64 SYS_arch_prctl syscall NUMBER, not an operation argument, so
  both diagnostics now read
  `syscall(SYS_arch_prctl /* 158 */, ARCH_{REQ,GET}_XCOMP_PERM, ...)`.
  Raised by CodeRabbit on #275 and correct on the merits.

Gates, run locally on the merged tree: `cargo fmt --check` clean,
`cargo check --lib --features std` clean, `cargo clippy --lib --features
std` no errors, and `cargo run --example geostrophic_stencil` green with
all 4 pre-registered bars PASS on the scalar-fallback path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CcpLeEC3XK8Eye53GKBVvi

Author: claude

.claude/AMX_GOTCHAS.md

@@ -281,9 +281,11 @@ directly (not just CPUID) — each a few lines of stable asm (Gotcha 4/5 has the
   0 (a generic model name like `Xeon @ 2.80GHz` is the tell).
 - `_xgetbv(0)` XCR0 b17/b18 (TILECFG/TILEDATA) — if **0**, the OS never enabled
   tile XSTATE; AMX cannot run, full stop.
-- `arch_prctl(158, ARCH_REQ_XCOMP_PERM, XTILEDATA=18)` — `-95 / -EOPNOTSUPP`
+- `syscall(SYS_arch_prctl /* 158 */, ARCH_REQ_XCOMP_PERM, XFEATURE_XTILEDATA=18)`
+  — `-95 / -EOPNOTSUPP`
   means the kernel REFUSES the grant; **no byte-call can force it**.
-- `arch_prctl(158, ARCH_GET_XCOMP_PERM, &mask)` — XTILEDATA (b18) must be in mask.
+- `syscall(SYS_arch_prctl /* 158 */, ARCH_GET_XCOMP_PERM, &mask)` — XTILEDATA (b18)
+  must be in mask.
 
 Observed once on one provisioning (kernel 6.18.5 guest): CPUID=0 **and** XCR0
 b17/b18=0 **and** REQ=`-EOPNOTSUPP` → genuinely unavailable. `detect_amx()`

src/simd.rs

@@ -645,12 +645,11 @@ pub use crate::hpc::cascade;
 // `backend::gemm_bf16` (portable scalar / NEON / wasm-SIMD paths).
 #[cfg(all(feature = "std", target_arch = "x86_64"))]
 pub use crate::hpc::amx_matmul::{amx_available, matmul_i8_to_i32};
-// BF16 16×16 tile GEMM — AMX `TDPBF16PS` when `amx_available()`, `F32x16` FMA
-// fallback otherwise. Same W1a rule: reach it via `ndarray::simd::*`, never
-// `crate::hpc` directly (the one-time `arch_prctl(158)` XTILEDATA grant is gated
-// inside `amx_available()`, which this dispatcher calls before any tile op).
-#[cfg(feature = "std")]
-pub use crate::hpc::bf16_tile_gemm::bf16_tile_gemm_16x16;
+// (The BF16 tile GEMM this branch re-exported unaliased is already surfaced
+// twice below — `bf16_tile_gemm_16x16` from `simd_ops` (the pure polyfill
+// kernel) and `bf16_tile_gemm_16x16_amx` from `hpc` (the tile-dispatching
+// wrapper). A third, unaliased `hpc` re-export collided with the `simd_ops`
+// name; dropped at the 2026-08-11 merge in favour of master's two-name split.)
 // Runtime-dispatch trampolines (`simd_runtime`, feature = "runtime-dispatch")
 // surfaced through the canonical `ndarray::simd::*` namespace — the W1a
 // consumer invariant is "all SIMD from `ndarray::simd`", so consumers that
@@ -682,7 +681,8 @@ pub use crate::simd_runtime::matmul_i8_to_i32;
 #[cfg(all(feature = "std", target_arch = "x86_64"))]
 pub use crate::hpc::bf16_tile_gemm::{
     bf16_tile_gemm_16x16 as bf16_tile_gemm_16x16_amx, bf16_tile_gemm_16x16_packed, bf16_tile_gemm_tier, PackedBf16B,
-};// CPU-generation detection (cached): SPR / EMR / GNR / Sierra Forest. Lets a
+};
+// CPU-generation detection (cached): SPR / EMR / GNR / Sierra Forest. Lets a
 // consumer report which silicon a run landed on and distinguish "no AMX
 // silicon" from "AMX present but not OS-enabled" — both surface via `amx_report`.
 #[cfg(target_arch = "x86_64")]