Function spawn 

pub fn spawn(spec: AuditSinkSpec) -> AuditForwarder

Spawn a forwarder task that consumes events from rx and sends them to spec. Returns the AuditForwarder handle the daemon writes to.

On task exit (channel closed by all senders dropping), the task returns silently. There is no graceful drain — pending events in the channel are dropped at shutdown. Audit durability is guaranteed by the local hash-chained log, not by the forwarder.