feat(select): fd arms in select + timed fd waits (RFC 008 phase 1)

FdArm composes fd readiness with channel arms on one wait epoch.
Selectable grows fallible sel_register and an eager-cleanup hook;
losing/stop-unwound/timed-out fd arms are unregistered (waiters entry
+ kernel ONESHOT) so the fd is never poisoned. try_select /
try_select_timeout surface registration errors (EBADF, EMFILE,
AlreadyExists) instead of RFC 008's permanently-ready lean, which
would busy-loop a healthy-but-unregistrable fd; select/select_timeout
stay infallible for channel-only arms. Adds wait_readable_timeout /
wait_writable_timeout as one-arm selects.

Known benign race (pre-existing, slightly widened): a queued FdReady
racing the cleanup DEL can spuriously wake a fresh waiter on that fd;
absorbed by select's defensive re-loop. Fixable by epoch-stamping
completions.

src/lib.rs

@@ -45,7 +45,10 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 // Public API re-exports
 // ---------------------------------------------------------------------------
 
-pub use channel::{channel, select, select_timeout, Receiver, RecvError, RecvTimeoutError, Selectable, Sender};
+pub use channel::{
+    channel, select, select_timeout, try_select, try_select_timeout, Receiver, RecvError,
+    RecvTimeoutError, Selectable, Sender,
+};
 pub use gen_server::{CallError, CallTimeoutError, CastError, GenServer, ServerBuilder, ServerCtx, ServerRef, Watcher};
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
@@ -55,7 +58,8 @@ pub use registry::{name_of, register, unregister, whereis, RegisterError};
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{
     block_on_io, request_stop, run, self_pid, sleep, spawn, spawn_under, wait_readable,
-    wait_writable, yield_now, JoinError, JoinHandle,
+    wait_readable_timeout, wait_writable, wait_writable_timeout, yield_now, FdArm, JoinError,
+    JoinHandle,
 };
 pub use supervisor::{ChildSpec, OneForOne, Restart, Signal, Strategy};