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

Überprüfe die Inline-Signaturannotationen aus #1738 und die aufgeführten Template-Einstiegspunkte, einschließlich BasicEnv::PostFinalizer, Object::AddFinalizer, External::New, Buffer::New und der ThreadSafeFunction-APIs. Bestimme die erforderlichen Einschränkungen für jeden Finalizer-, Callable-, Getter-, Setter- und Callback-Kandidaten. Als abgeschlossen gilt die Aufgabe, wenn C++20 Concepts die dokumentierten Signaturen erzwingen und Diagnosen an der Aufrufstelle erzeugen.