Heaps and k-way merge

All inner feeds are already sorted nondecreasing. Return every value sorted, retaining duplicates. Empty outer/inner vectors are valid; no sortedness validation is required. Prototype concatenate/sort; final one heap frontier per active feed. Preserve outer and inner vectors.

Public signatures:

Vec<Int> prototype(const Vec<Vec<Int>>& feeds);
Vec<Int> solve(const Vec<Vec<Int>>& feeds);

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.