Buffer emissions for async iterable integrity in Poll

[afshin]

This PR updates Poll#[Symbol.asyncIterator] with a buffered implementation that connects to the ticked signal for each iterator returned to a client, ensuring rapid schedule() calls no longer silently drop intermediate states.

Previously, the iterator read this.state directly on each yield. When schedule() was called multiple times before the consumer resumed, this._state was overwritten and only the last state was visible.

The implementation in this PR pushes every ticked emission into a per-iterator queue and yields from it in order. When the queue is empty, it awaits this.tick as a wake-up signal. Cleanup is handled in a finally block to guarantee the signal listener is disconnected on break, return, throw, or disposal.

This problem does not exist in Stream from @lumino/signaling because stream's async iterator is based off of a promise chain instead of being a push-based state machine like Poll. The yield paradigm is pull-based. So to bridge this gap naively like the original implementation in Poll meant silently dropping emissions that disappear with no error because the poll state is being overwritten before the next yield. Effectively, then, Stream has a functionally unbound queue (a linked list of promises) and Poll has a bound queue of 1000 emissions that can pile up if a client consumes yielded values slowly.

No new API surface, no type changes, no effect on Throttler/RateLimiter.

Normal single-tick-at-a-time polling behaves identically.

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The added tests in the polling test suite will fail (timeout) on the main branch but will pass with these changes.

[Copilot]

Pull request overview

Updates Poll#[Symbol.asyncIterator] to buffer ticked emissions per iterator, preventing rapid schedule() calls from overwriting intermediate states before a consumer resumes iteration.

Changes:

• Reworked Poll#[Symbol.asyncIterator] to enqueue each ticked state and yield in FIFO order, with finally-based disconnect cleanup.
• Added tests validating no dropped states under rapid schedule() calls, correct behavior after disposal, and unchanged behavior for one-tick-at-a-time polling.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/polling/src/poll.ts	Implements per-iterator buffering of ticked emissions and ensures signal disconnection on iterator termination.
packages/polling/tests/src/poll.spec.ts	Adds regression and behavior tests covering rapid scheduling, disposal, and baseline iteration behavior.

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