Execute every unprocessed child's request AA in FIFO order, then prune processed children so the tree doesn't grow unbounded over a long session.

Tier 2 fire-and-forget writer — FIFO children-as-vehicle queue (Architecture C, continuous-server). Mirrors ApiQueueTask, minus the 3-slot pool and response routing: side effects have no response to deliver, so each request executes inline on this single thread in append order (serialised, must-complete: mark watched, playstate, favorite, …).

Why children instead of the old shared 'request' field: SceneGraph coalesces port events when a field is written rapidly from the render thread. Two back-to-back SubmitSideEffect() calls could merge into one event, and the second 'request' write clobbered the first before this thread read it — silently dropping a POST (issue #744: the capabilities POST lost behind the cold-launch /pair POST). Children are immune: even if wake-up events coalesce, we read ALL pending children from the node tree, so no request is ever lost regardless of timing.