Return async iterables from commands

Describe the problem

If a command kicks off a long-running task, and you want to get progress updates, you basically have to return an ID and use that to call a query.live. It involves a bunch of book-keeping on both sides of the network divide.

In some respects this is the right way to deal with it — you can use that live query in different contexts, and if it's a really long running task then you can come back to it after navigating somewhere else — but for a lot of smaller jobs, being able to get progress reports directly from the command is the sweet spot.

Describe the proposed solution

Something like this — essentially, adding async iterables to the list of things that commands can return:

export const insertLargeThing = command(v.object(...), async function* (data) {
  yield { progress: 0 };

  const task = startTask(data);

  while (!task.done) {
    yield { progress: task.progress };
    await sleep(50);
  }
});

<button
  onclick={async () => {
    for await (const task of insertLargeThing(...)) {
      progress = task.progress;
    }

    done = true;
  }}
>
  insert large thing
</button>

Alternatives considered

not doing this

Importance

nice to have

Additional Information

No response

Start by tracing how commands are currently invoked and how their return values cross the network boundary; the issue does not name specific files or tests. Define the async-iterable behavior for progress values, completion, and the client-side for-await usage, then add coverage for the proposed command and Svelte examples.