Enforce callback signatures with C++ Concepts

#1738 documented the required signature of the templated finalizer callback parameters as inline comment hints.

These are human-only annotations. The compiler does not enforce them, so a mismatched callable fails deep inside the template instantiation with an unhelpful error.

Once C++20 is the minimum supported standard, we should replace these hints with C++ Concepts that constrain the Finalizer/FinalizerType template parameters. This gives compiler-enforced contracts and clear diagnostics at the call site.

Finalizer candidates:

• BasicEnv::PostFinalizer
• Object::AddFinalizer
• External::New
• Buffer::New / Buffer::NewOrCopy
• ThreadSafeFunction::New
• TypedThreadSafeFunction::New

Callback candidates:

• Function::New — the JS callback (Callable)
• PropertyDescriptor::Accessor — Getter / Setter
• PropertyDescriptor::Function — Callable
• ThreadSafeFunction::BlockingCall / NonBlockingCall — Callback
• TypedThreadSafeFunction::BlockingCall / NonBlockingCall — Callback
• TypedThreadSafeFunction — the CallJs callback

Examinez les annotations de signature inline de #1738 et les points d’entrée de templates indiqués, notamment BasicEnv::PostFinalizer, Object::AddFinalizer, External::New, Buffer::New et les API de ThreadSafeFunction. Déterminez les contraintes nécessaires pour chaque candidat Finalizer, Callable, Getter, Setter et Callback. Le travail est considéré comme terminé lorsque les Concepts C++20 imposent les signatures documentées et produisent des diagnostics au niveau de l’appel.