Module waker 

Custom RawWakerVTable implementation.

Each Waker holds an Arc<TaskHeader> cast to a raw *const (). The four vtable functions implement the RawWaker contract:

function	action
clone_waker	Arc::clone — increments refcount
wake	schedule task, consume (decrement) Arc
wake_by_ref	schedule task, keep Arc alive
drop_waker	Arc::from_raw then drop — decrements refcount

Safety contract: the data pointer is always a valid Arc<TaskHeader> that was created via Arc::into_raw. All four functions restore it to an Arc before performing any operation, maintaining the reference count correctly.