State<Ref> rebinds (refA -> refB) go through process's own diff (same
handler type matches both), not retract -- exactly the TagHandler
precedent. retract only fires when the slot stops being a Ref at all.
Both paths converge on old:Set(nil). Non-nilable Ref<T> remains a valid
"read a settled value" use case; callers opt into Ref<T?> explicitly for
Store/Modifier slots. Unbinding is independent of Instance Destroy --
that's Effect's job, not Ref's.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
