Function spawn 

pub fn spawn(
    bus: Arc<EventBus>,
    registry: Arc<Mutex<PluginRegistry>>,
    format_name: String,
) -> JoinHandle<Result<()>>

Spawn the subscriber task. Returns a join handle the orchestrator awaits at shutdown so the BuildEnd event is fully drained.

format_name must already exist in registry.output_formatter_index — scheduler::run validates this before emitting BuildStart, so a missing entry here means we lost a race against a concurrent registry mutation (impossible in single-run orchestration). We drop events silently in that case and exit on BuildEnd.