feat(channel): select_timeout — bounded select on a stateless stamped timer arm

The deadline is one more stamped waker on the select's wait epoch: a unit
TimerTarget whose on_timeout is a bare unpark_at. Nothing registers in any
arm for it, so there is nothing to cancel or leak — an arm winning leaves
the timer entry to die at its epoch CAS; the timer winning leaves the arms'
registrations to self-clean like any select loser's. Classification is pure
channel state (wakes are precise): some arm ready -> Some(first, priority
order); none -> the timer was the only remaining stamped waker -> None.
Message-first on a raced deadline, same as recv_timeout.

Registration pass factored into register_arms, which owns the retire_wait
obligation on the ready-now exit for both entry points.

src/lib.rs

@@ -45,7 +45,7 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 // Public API re-exports
 // ---------------------------------------------------------------------------
 
-pub use channel::{channel, select, Receiver, RecvError, RecvTimeoutError, Selectable, Sender};
+pub use channel::{channel, select, select_timeout, 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};