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

检查 #1738 中的内联签名注解以及列出的模板入口点，包括 BasicEnv::PostFinalizer、Object::AddFinalizer、External::New、Buffer::New 和 ThreadSafeFunction APIs。确定每个 Finalizer、Callable、Getter、Setter 和 Callback 候选项所需的约束。完成标准是由 C++20 Concepts 强制执行文档中规定的签名，并在调用点生成诊断信息。