feat(hpc/reductions): SIMD-dispatched sum/mean/max/min/argmax/nrm2 (#122, sprint A10)

* feat(hpc): add SIMD reductions dispatcher (sum/mean/max/min/argmax/argmin/nrm2)

* fix(reductions): mean_f32/f64 return Option per A10 spec

Spec for sprint A10 calls for:
  pub fn mean_f32(s: &[f32]) -> Option<f32>;     // None on empty

Previously mean_f32/mean_f64 panicked on empty input. This change
returns None for empty slices, matching argmax_f32 / max_f32 / min_f32
which already use the Option convention.

Tests:
- mean_f32_empty_is_none — verifies None on empty input
- mean_f64_empty_is_none — verifies None on empty input
- mean_f32_basic — non-empty case via .expect()

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: AdaWorldAPI

src/hpc/mod.rs

@@ -25,6 +25,7 @@ pub mod simd_dispatch;
 pub mod blas_level1;
 pub mod blas_level2;
 pub mod blas_level3;
+pub mod reductions;
 pub mod statistics;
 pub mod activations;
 pub mod hdc;