…U choice forever

simd_dispatch.rs (300+ lines, 7 tests):

  SimdDispatch: struct of function pointers, frozen at first access via LazyLock.
  Each field is a fn pointer to the best available implementation for this CPU.
  After initialization: one pointer deref + one indirect call. Zero branching.

  SimdTier enum: Avx512 / Avx2 / Sse2 / Scalar / WasmSimd128 (future).
  Selected once based on simd_caps() detection. Frozen forever.

  Before: if simd_caps().avx512f { avx512_fn() } else { scalar_fn() }  → ~1ns + branch
  After:  (SIMD_DISPATCH.fn_ptr)(args)                                  → ~0.3ns, no branch

  Dispatch targets (6 free functions across 4 modules):
    byte_scan:    byte_find_all, byte_count (AVX-512 / AVX2 / scalar)
    distance:     squared_distances_f32 (AVX2 / scalar)
    nibble:       nibble_unpack, nibble_above_threshold (AVX2 / scalar)
    spatial_hash: batch_sq_dist (AVX2 / scalar)

  NOTE: aabb.rs and cam_pq.rs dispatch on &self methods (not free functions)
  so they keep inline simd_caps() branching. The dispatch table covers
  the free function hot paths.

  Visibility: internal SIMD functions promoted from pub(super)/private
  to pub(crate) so the dispatch table can reference them as fn pointers.

  The 8 existing per-call dispatch sites in nibble/byte_scan/distance/
  spatial_hash/aabb/cam_pq still work — the dispatch table is additive.
  Consumers can migrate to simd_dispatch().fn_ptr() incrementally.

  TODO (separate PR): Rust 1.94 stabilized safe #[target_feature] on
  safe functions. The `unsafe` on SIMD functions is legacy debt that
  should be removed. The dispatch wrappers currently bridge this with
  SAFETY comments; once unsafe is removed, the wrappers simplify to
  direct function pointer assignment.

https://claude.ai/code/session_01Y69Vnw751w75iVSBRws7o7