Add passive signal observers

What is the problem this feature will solve?

Node currently exposes signals through process.on('SIGINT') / process.on('SIGTERM'), but that API combines two very different needs:

• observing that a signal happened
• taking ownership of shutdown behavior

That coupling creates avoidable ecosystem friction. Many libraries only need best-effort cleanup when a process is interrupted, for example restoring terminal state, showing the cursor again, or stopping a spinner. Today, the only way to do that is to install a real signal handler, which changes global process semantics, suppresses default behavior, and can interfere with app-defined handlers.

What is the feature you are proposing to solve the problem?

Node should add a passive API such as:

process.observeSignal('SIGINT', () => {
	restoreTerminalState();
});

A passive observer would be notified when the signal arrives, but would not count as a handler, would not suppress Node's default behavior, and would not affect app ownership of signal handling. Apps that want to intercept or overrride shutdown would continue to use process.on(...).

This would give Node a clean separation between "tell me this happened" and "I am handling this". It solves a real problem for CLI and terminal libraries, reduces handler conflicts, preserves backward compatibility, and makes signal behavior more predictable across the ecosystem.

What alternatives have you considered?

Considered alternatives:

1. Reuse process.on() / process.once()
   Rejected because it does not separate observation from handling. Libraries still become real signal handlers and can change global process behavior.

2. Rely on exit hooks like exit / beforeExit
   Rejected because they are not signal-specific and are too late or inconsistent for cleanup that should happen when the signal is delivered.

3. Tell libraries to avoid signals entirely
   Rejected because it pushes the problem onto every app and leaves reusable CLI libraries without a clean way to do best-effort cleanup.

4. Add ordering, priority, or metadata to existing signal handlers
   Rejected because it may reduce conflicts, but it still treats passive observers as active handlers. Th core problem remains.

首先审查 Node 现有的 process.on('SIGINT') 和 process.on('SIGTERM') 信号处理，以及 issue 中描述的 exit 和 beforeExit 替代方案。定义被动观察者必须如何区别于真正的处理程序，包括保留默认行为和应用定义的所有权。只要所提议的 API 及其信号语义已经明确规定到足以实现的程度，即视为完成。