StopSignal interception in tick_all set stopping=true but did not push
to the deaths vector, so phase 5b watch notifications never fired for
externally-stopped actors (via rt.stop_actor()). Now pushes
(addr, ExitReason::Stopped) to deaths, consistent with the
ctx.stop_self() path.
Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski
a9835f27ad