Strings, Unicode, and standard containers

Prototype: count every input byte into an ordered map<unsigned char,Int>, including NUL and high bytes. Final: strictly decode UTF-8 into owned Text. Accept every Unicode scalar value, including NUL and U+10FFFF; reject invalid leading/continuation bytes, truncation, overlong forms, surrogates, and out-of-range scalars with invalid_argument. Empty bytes decode to empty. Preserve input; no normalization or replacement characters.

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::map<unsigned char,Int> prototype(std::string_view bytes);
inline Text solve(std::string_view bytes);

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.