feat(channel): select — ready-index wait over multiple receivers

select(&[&dyn Selectable]) -> usize registers (pid, epoch) in every arm
under one wait epoch, parks once, and returns the first ready index in
documented priority order (BEAM-style; no fairness promise). A closed arm
counts as ready — and stays ready forever, so callers drop it from the set
once observed. Losing arms need no cancellation pass: the winning wake
consumed the epoch, so their registrations die at their wakers' failed CAS
or are overwritten by the receiver's next wait on that channel (the
single-receiver debug_asserts relax to 'none or own pid' accordingly).

The one genuinely new protocol piece is the no-park exit: returning with an
arm ready at registration time leaves earlier arms holding LIVE-epoch
registrations, whose wakes could fault a later one-shot park as a pending
notification. scheduler::retire_wait closes it — bump the epoch (in-flight
wakes die at their CAS), eat a notification that already landed
(StateWord::clear_notify), then re-observe the stop flag, in that order.
Proved by the new loom theorem retire_eats_late_arm_notification; the
integration probes in tests/select.rs fire stale loser-arm wakes and assert
a subsequent sleep's one-shot park holds its full duration.

src/lib.rs

@@ -45,7 +45,7 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 // Public API re-exports
 // ---------------------------------------------------------------------------
 
-pub use channel::{channel, Receiver, RecvError, RecvTimeoutError, Sender};
+pub use channel::{channel, select, Receiver, RecvError, RecvTimeoutError, Selectable, Sender};
 pub use gen_server::{CallError, CallTimeoutError, CastError, GenServer, ServerRef};
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};