std::link_monitorActor monitor / link handle helpers.
Import std::link_monitor when you want to name MonitorRef or
LinkError explicitly. The monitor(actor) builtin returns MonitorRef;
the link(actor) builtin returns Result<(), LinkError>.
v0.5 link/monitor builtins return through the Cluster 2 composite-return
spine, which lands in v0.6. The runtime symbols exist (hew_actor_link,
hew_actor_monitor), but the user-source surface fails closed today. See
commit 1698840f for the explicit deferral.
LinkErrorError returned by link(handle).
AlreadyLinked is an idempotent non-fatal condition โ callers may
treat it as Ok(()). TargetDead is returned when the target actor
has already exited; the caller's exit handler will fire immediately.
AlreadyLinkedTargetDeadMonitorRefA handle to an active monitor registration.