Numeric Algorithms：accumulate、reduce、iota、scan | STL

本章覆盖：`<numeric>`；`accumulate`；initial value type；integer overflow / type deduction risk；`reduce`；execution policy 高层认知；associativity requirement；`iota`；`inner_product`；partial sum；inclusive / exclusive scan；parallelization 语义边界；金额 / 浮点累加的数值稳定性问题。