Numeric Algorithms (accumulate, inner_product, iota)

The <numeric> header collects a small family of algorithms for folding, scanning, and generating numeric sequences: summing a range, computing a dot product, building prefix sums, and filling a range with successive values. They sit alongside the <algorithm> algorithms but stay in a separate header because they were originally aimed at numeric work. This chapter covers each one with its common pitfalls, the C++17 parallel-friendly counterparts (reduce, transform_reduce, the scans), and the small math helpers gcd and lcm. The general-purpose <algorithm> functions (sort, find, transform, and friends) live in the next set of chapters.