Floyd–Warshall all-pairs shortest paths

Return n*n directed minimum-cost matrix with zero diagonal and nullopt unreachable; keep cheapest parallel edge. n=0 yields empty. Negative n, invalid endpoints, or ANY negative cycle throws invalid_argument. Prototype Bellman-Ford per source; final Floyd-Warshall O(V^3)/O(V^2). Preserve edges.

Public signatures:

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