The composer's single primary action now stops a running continuable
child: Session.cancel() routes a continuable address through
subagent.interrupt (ordinary sessions keep session.cancel; one-shot
addresses stay uncancellable), and InputBar switches the same Send button
to Stop while a continuable child runs. A running parent-offline child
keeps the default composer with disabled input so that Stop stays
reachable; the read-only takeover returns once it stops.
Refs #1535
