[2026-03 LWG Motion 3] P3690R1 Consistency fix: Make simd reductions SIMD-generic

[jensmaurer]

P3690R1 (Consistency fix: Make simd reductions SIMD-generic)

Talk to @mattkretz for LaTeX sources.

[mattkretz]

https://github.com/mattkretz/wg21-papers/blob/master/P3690_make_simd_reductions_simd-generic%2Fwording.tex