Checkpoint: Serializer

Serializer is the exam because it requires all three template forms in one small system:

• the primary handles anything std::to_string accepts
• the full specialization for std::string stops numbers-only behavior from mangling text
• the partial specialization for std::vector<T> recurses: element serialization goes back through the chooser, so nested vectors compose

The recursion is the concept being tested: Serializer<vector<T>>::to calls Serializer<T>::to without knowing what T is. That single line is how the whole standard library composes.

Watch the exact output format: [1,2,3] — no spaces, brackets included. Off-by-one separators are the classic specialization bug.