ENH: Convert arithm_fp from C universal intrinsics to C++ using Highway (#17)

* ENH, SIMD: Initial implementation of Highway wrapper

A thin wrapper over Google's Highway SIMD library to simplify its interface.
This commit provides the implementation of that wrapper, consisting of:

- simd.hpp: Main header defining the SIMD namespaces and configuration
- simd.inc.hpp: Template header included multiple times with different namespaces

The wrapper eliminates Highway's class tags by:
- Using lane types directly which can be deduced from arguments
- Leveraging namespaces (np::simd and np::simd128) for different register widths

A README is included to guide usage and document design decisions.

* SIMD: Update wrapper with improved docs and type support

  - Fix hardware/platform terminology in documentation for clarity
  - Add support for long double in template specializations
  - Add kMaxLanes constant to expose maximum vector width information
  - Follows clang formatting style for consistency with NumPy codebase.

* SIMD: Improve isolation and constexpr handling in wrapper

  - Add anonymous namespace around implementation to ensure each
    translation unit gets its own constants based on local flags
  - Use HWY_LANES_CONSTEXPR for Lanes function to ensure proper
    constexpr evaluation across platforms

* Update Highway submodule to latest master

* SIMD: Fix compile error by using MaxLanes instead of Lanes for array size

  Replace hn::Lanes(f64) with hn::MaxLanes(f64) when defining the index array
  size to fix error C2131: "expression did not evaluate to a constant". This error
  occurs because Lanes() isn't always constexpr compatible, especially with
  scalable vector extensions. MaxLanes() provides a compile-time constant value
  suitable for static array allocation and should be used with non-scalable SIMD
  extensions when defining fixed-size arrays.

* Convert arithm_fp to highway.

---------

Co-authored-by: Sayed Adel <seiko@imavr.com>

Author: 2 people

numpy/_core/meson.build

@@ -921,13 +921,14 @@ umath_gen_headers = [
 foreach gen_mtargets : [
   [
     'loops_arithm_fp.dispatch.h',
-    src_file.process('src/umath/loops_arithm_fp.dispatch.c.src'),
+    src_file.process('src/umath/loops_arithm_fp.dispatch.cpp.src'),
     [
       [AVX2, FMA3], SSE2,
       ASIMD, NEON,
       VSX3, VSX2,
       VXE, VX,
       LSX,
+      RVV,
     ]
   ],
   [