Copy, move, and the rule of zero

Prototype takes Packet by value, appends extra, returns the owned result; an lvalue caller remains unchanged. Final drains exactly count packets from the front of source into a returned vector in order. Require 0<=count<=source.size(), otherwise invalid_argument before mutation. Transfer each nonempty inner vector allocation without copying Int elements; remaining packets preserve order/content. count zero changes nothing; draining all empties source. Return values own independent contents. Outer Packet references may be invalidated.

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).

struct Packet {Vec<Int> values;bool operator==(const Packet&)const=default;};
inline Packet prototype(Packet packet,Int extra);
inline Vec<Packet> solve(Vec<Packet>& source,Int count);

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.