Draft - Code Cleanup and New Function

[shsahiti]

Summary

Cleans up namespace qualification across linear_algebra.hpp, memory.hpp, random.hpp, and soa.hpp (dropping redundant xpu:: prefixes while keeping required ones, fixing a few that were incorrectly stripped). Fixes an unclosed sincos signature issue in math.hpp.

Adds three new parallel reduction functions in a new reduce.hpp header, built on the existing parallel_reduce_sum infrastructure:

• accumulate
• average
• geometric_mean