Returns the Spawner that produced this thing, or null when
unset OR when the previous Spawner has destructed (R2.3
self-heal). The clear-on-destroy fold happens here so consumers
never receive a destroyed ref.
Setter for the back-pointer. Normal callers don't invoke this
directly — it's wired by Spawner.trackSpawn /
Spawner.untrackSpawn. Public so the symmetric Spawner-side
helper can reach it.
Public shape provided by SpawnedMixin.