Values, initialization, and control flow

Prototype: classify an Int as exactly "negative", "zero", or "positive". Final: parse a nonempty ASCII decimal string with an optional leading + or -, at least one digit, no whitespace or separators, and no trailing data. Leading zeroes are valid. Return nullopt for lexical or Int range failure; do not throw. Accept both Int endpoints and negative zero. Do not call a library integer parser in this exercise.

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 std::string prototype(Int value);
inline Opt<Int> solve(std::string_view input);

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.