Expand description
Backend-agnostic racing combinators (replace tokio::select! at the
drive-loop sites). Poll-based and unbiased: both futures are polled on
every wake; whichever completes first wins.
Futures are taken by value — callers pass &mut of core::pin::pin!(..)
bindings (&mut Pin<&mut F> is a Future and is Unpin).
Cancel-safety matches tokio::select!: when the combinator completes on
one arm, the other arm’s future is dropped (work on it stops).