Sorting and two pointers

Both stages return the lexicographically smallest ascending value pair summing to target from distinct positions, or nullopt. Negatives and duplicates are valid. Prototype tests all pairs; final sorts a copy then scans inward. Preserve input.

Public signatures:

Opt<std::pair<Int,Int>> prototype(const Vec<Int>& weights,const Int& target);
Opt<std::pair<Int,Int>> solve(const Vec<Int>& weights,const Int& target);

Compile and run from this directory:

c++ -std=c++20 -O2 -Wall -Wextra -pedantic -pthread main.cpp -o practice
./practice --stage prototype
./practice --stage final

Edit exercise.hpp. Both stages start unfinished and must report failures until implemented. Keep main.cpp as the executable contract. The runner returns nonzero for a failed check, an unexpected exception, an invalid stage, or an empty suite. C++20 and only the standard library are required.