true on success, false if no loop or already attached.
Thread_Affinity: Must be called from the loop thread.
FiberScheduler.attachToLoop
bool attachToLoop() @trusted nothrowAttach the scheduler to the EventLoop's prepare phase.
Registers a prepare-phase watcher that drains the ready queue on every loop iteration. The prepare phase fires before polling and timer expiration, ensuring spawned fibers execute promptly. Call detachFromLoop() to remove the watcher.