Iterators, algorithms, ranges, and invalidation

Prototype returns sorted unique values without mutating input. Final merges values and incoming, removes values below minimum, sorts ascending, deduplicates, and commits into values. Incoming is preserved when distinct, and MAY ALIAS values; self-merge must behave as though both inputs were read before mutation. Return void. No dangling iterator use, and no input sortedness assumption. An empty result is valid.

C++ contract conventions: numeric values use signed Int (long long); all inputs and intermediate arithmetic are within its representable range. Opt<T> means std::optional<T> and absence is std::nullopt; Vec<T> means std::vector<T>. Invalid documented arguments throw std::invalid_argument. Inputs passed by const reference remain caller owned. Returned containers and records own their contents. No runtime type coercion is required; C++ static types exclude many dynamic-language type errors. Unless a lesson states otherwise, string ordering is lexicographic over Unicode code points (Text) or UTF-8 identifiers (std::string).

inline Vec<Int> prototype(const Vec<Int>& values);
inline void solve(Vec<Int>& values,const Vec<Int>& incoming,Int minimum);

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.