Binary trees and level-order traversal

Input is a null root or Tree(name,left,right), with unique names, no cycles and no shared child nodes. Return Text names grouped by depth left-to-right. Prototype rescans per depth; final queue O(n) time/O(w) auxiliary space. Preserve structure and names. Public Tree shared owners replace recursive tuple input.

Public signatures:

Vec<Vec<Text>> prototype(const std::shared_ptr<Tree>& root);
Vec<Vec<Text>> solve(const std::shared_ptr<Tree>& root);

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.