feat(channel): recv_timeout - bounded receive on the WaitTimeout machinery

The per-recv deadline the roadmap deferred, built exactly the way
Mutex::lock_timeout already works: register the wait with a per-wait
seq, arm a timer::Reason::WaitTimeout with the channel inner (now a
TimerTarget) as the target, park. On expiry on_timeout cancels the wait
only if that same seq is still parked; satisfied or abandoned waits
leave a stale heap entry that no-ops on seq mismatch, per the
no-cancellation convention in timer.rs.

Race resolution is message-first: a send that lands by the time the
woken receiver runs is delivered even if the deadline had also passed.
Closure is reported as RecvTimeoutError::Disconnected, keeping timeout
and server/sender death distinguishable for callers (gen_server call
timeout builds on exactly this distinction next).

The timer is armed outside the channel critical section (the timers
lock must never nest under a Channel-class lock); the unpark race this
opens is absorbed by the RunningNotified protocol.

Tests: immediate delivery, actual timeout (with elapsed check), prompt
wake on send, Disconnected on close, zero-duration poll, post-timeout
seq isolation (stale entry must not cancel later waits), and a
4-scheduler mixed-outcome run (12 fed / 12 timed out).

src/lib.rs

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