Backtracking and pruning

Choose one Text per inner vector with no repeated Text. Return lexicographically smallest assignment in original position order, or nullopt. Duplicate choices add no alternatives. [] returns an engaged empty vector; an empty inner list makes assignment impossible. Prototype enumerate complete combinations; final sorted choices with early rejection/rollback. Preserve all input vectors.

Public signatures:

Opt<Vec<Text>> prototype(const Vec<Vec<Text>>& allowed);
Opt<Vec<Text>> solve(const Vec<Vec<Text>>& allowed);

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.