The scheduler integration (#44810) removed the special handling for
realtime audio tasks that spawn them on a dedicated thread. This caused
a panic in the Mac dispatcher when RealtimeAudio priority was passed to
the regular dispatch path.

This restores the original behavior: RealtimeAudio tasks are spawned on
a dedicated thread via dispatcher.spawn_realtime(), using a bounded
channel to send runnables to it.

Release Notes:

- N/A