Target Sum Pair Finding

Perforce Helix Core tooling may need to identify two numeric change metrics whose combined value matches a requested threshold. Given an array of integers and a target integer, return any pair of distinct elements whose values add up to the target. Return -1 if no such pair exists.

Formal Specification

Implement pair_sum(nums, target).

• Input: nums, a list of integers, and target, an integer.
• Output: A two-element list containing the values of two distinct array elements whose sum equals target, or the integer -1 when no pair exists.
• An element may not be used twice, but duplicate values at different indices may form a valid pair.
• If multiple pairs exist, return any one of them.