Jetstream: support bounded queue-preserving backpressure

[simnaut]

Package

@atcute/jetstream 2.0.1

Problem

JetstreamSubscription delivers WebSocket messages through
@mary-ext/event-iterator, whose queue is observable but unbounded. A sequential
consumer that awaits durable I/O can therefore accumulate complete Jetstream
records faster than it drains them.

Closing the iterator with return() is not a safe pressure mechanism because it
clears the queue. Reconnecting from the application's durable processed cursor
then replays the cleared range; under sustained pressure this can cycle without
catching up. Fresh subscriptions using a URL array also intentionally roll back
ten seconds, which amplifies that starvation mode.

Production observations

In a Cloudflare Durable Object consumer:

• Unbounded intake reached roughly 177–185 MiB memory P90.
• Polling iterator.queued, calling return(), and reconnecting reduced memory
  but caused discontinuous readiness and cursor starvation.
• A queue-preserving prototype that closes only socket intake at a high-water
  mark, drains the existing iterator, and resumes the same endpoint at a
  low-water mark preserved cursor continuity.
• With 200/50 high/low watermarks, memory P90 fell to about 30 MiB. In an early
  repeated sample it processed 5,750 posts with 29 pause/28 resume cycles, no
  reconnect or run errors, and the queue never exceeded 200.

Suggested API

Would you consider first-class bounded buffering on JetstreamSubscription?

new JetstreamSubscription({
  url,
  maxQueuedEvents: 200,
  resumeQueuedEvents: 50,
  onBufferPause(stats) {},
  onBufferResume(stats) {},
})

Desired semantics:

1. Stop or close WebSocket intake at the high-water mark without clearing
   already admitted iterator events.
2. Continue ordered draining.
3. Resume the same endpoint at the newest admitted cursor when the low-water
   mark is reached, avoiding cross-endpoint rollback for a deliberate pressure
   pause.
4. Preserve normal multi-endpoint rollback for actual failover.
5. Expose current/max queued events, oldest queued age, pressure state, and
   pause/resume counts.
6. On consumer termination, retain the current return() behavior that clears
   the queue and closes the socket.

I can prepare a PR based on the validated prototype if this API direction fits
the project.