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/runtime.rs

@@ -553,6 +553,16 @@ impl RuntimeInner {
         slot.word.begin_wait(pid.generation())
     }
 
+    /// Retire the calling actor's current wait without parking on it: bump
+    /// the epoch (invalidating every in-flight registration-based wake),
+    /// then eat a notification that already landed. The caller MUST
+    /// re-check its stop flag afterwards — see `StateWord::clear_notify`.
+    pub(crate) fn retire_wait(&self, pid: Pid) {
+        let slot = self.slot_at(pid).expect("retire_wait: own slot vanished");
+        let _ = slot.word.begin_wait(pid.generation());
+        slot.word.clear_notify(pid.generation());
+    }
+
     fn unpark_inner(&self, pid: Pid, want: Option<u32>) {
         if let Some(slot) = self.slot_at(pid) {
             match slot.word.unpark(pid.generation(), want) {