Schedule a coroutine tied to this transformer's lifecycle.
The mux holds the task reference, awaits all scheduled tasks
during aclose() before calling afinalize(), and cancels
them on afail(). Authors don't need to track tasks or
implement the last-task-closes-the-log dance.
Requires a running event loop — call only under astream().
Set requires_async = True on the class so registration under
sync stream() fails fast with a clear message.
The coroutine to run. Its lifecycle is owned by the mux from this point on.
"log" (default) catches and logs any exception
the coroutine raises, so a single failure doesn't tear
down the run. "raise" lets the exception propagate
when the mux joins pendings, converting the close path
into the fail path.
