Re: [whatwg/streams] Proposal: `ReadableStream.withSafeResolvers()` (Issue #1359)

MattiasBuelens left a comment (whatwg/streams#1359)

> One possible refinement I’ve been considering is to drop the “safe” framing entirely and instead make the stream’s completion state observable.
> 
> Concretely, something along these lines:
> 
> ```js
> const resolvers = ReadableStream.withResolvers();
> const { stream, enqueue, close, error } = resolvers;
> ```

Or you could group those methods in a single object and perhaps... call it a controller? 😄

```javascript
const { stream, controller } = ReadableStream.withController();
controller.enqueue("a");
controller.close();
```

> ```js
> resolvers.completed;
> // → true if close() or error() has already been called, otherwise false
> ```
> 
> Here, `completed` would be exposed as a getter rather than a plain boolean value. If it were a data property, destructuring would snapshot the value and wouldn’t reflect subsequent state changes, which seems error-prone. A getter allows producers to deliberately check the current completion state without changing any existing semantics.
> 
> This approach wouldn’t suppress exceptions from `enqueue()`, `close()`, or `error()`, nor would it alter backpressure behavior. Those operations would still fail fast as they do today. The goal is simply to provide an explicit, opt-in way for producers to observe completion and avoid late enqueues intentionally, rather than relying on try/catch.
> 
> Exposing dynamic internal state via getters is also consistent with existing Streams APIs such as `stream.locked` or `controller.desiredSize`, which makes this feel like a natural extension rather than a special case.

This could be interesting, but we can then add it to the existing `ReadableStreamDefaultController` API instead. Something like:
* `controller.state`
* `controller.closed`
* ...

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