_attach_child now mirrors a pending parent stop with the same split
interrupt() uses for its own fan-out (hard -> hard_interrupt, soft ->
interrupt), so a redirect is not turned into a cancel on a child that was
attached late. _restore_parent_cancellation collapses to re-attaching the
rejected unit's children: the replay is the attach step's job now.
Test fixture: _Batch gained origin_session_history_delivery on main after
the salvaged PR was written.
Co-authored-by: illidan <noequal666@gmail.com>
teknium1