Topological sorting

Directed (u,v) means u before v. Return lexicographically smallest complete ordering or nullopt for any cycle. Duplicate edges count once. n=0 returns engaged empty vector. Negative n or endpoint out of range throws invalid_argument. Prototype lexicographic permutations; final indegrees/min-heap frontier. Preserve edges.

Public signatures:

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

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.