timer: send_after / cancel_timer message-delivery substrate

Add a cancellable message-delivery timer on the existing timer.rs min-heap,
the substrate the gen_server time idioms (idle/receive timeout, periodic tick,
debounce/backoff) need.

- Reason::Send { fire }: a type-erased delivery thunk. send_after (Pid<A>, via
  send_to) and send_after_named (Name<M>, via send) capture dest+msg and
  resolve the address on fire, not at arm time, so a dead target / restarted
  name is observed when it fires; a failed resolve or closed inbox is dropped
  (Erlang erlang:send_after semantics).
- Cancellation via an "armed" set keyed on the entry seq, exposed as an opaque
  TimerId. Only Send timers use it; Sleep/WaitTimeout keep their inert-stale
  behaviour untouched. pop_due fires a Send only while still armed and removes
  it, so cancel returns true iff it landed before the fire (the race signal).
  cancel is unscoped: any holder of the id can cancel (e.g. racing two servers
  and cancelling the slow path).
- peek_deadline contract documented as "<= true next deadline" so a future
  hierarchical timing wheel can back Timers without touching send_after or the
  scheduler idle path. call_timeout left on recv_timeout (blast radius).

Tests: Timers-level (fire/cancel/race/clear/ordering) plus scheduler-level
delivery for both Name<M> and Pid<A>, cancel-prevents-delivery, and silent
drop on unresolved name / dead pid. Green on rq-mutex/rq-mpmc/rq-striped.

src/scheduler.rs

@@ -9,7 +9,7 @@
 
 use crate::actor::current_pid;
 use crate::channel::Sender;
-use crate::pid::Pid;
+use crate::pid::{Name, Pid};
 use crate::runtime::{
     self, RuntimeInner, YieldIntent, RUNTIME,
 };
@@ -388,6 +388,57 @@ pub fn insert_wait_timer(
     });
 }
 
+// ---------------------------------------------------------------------------
+// send_after / cancel_timer — message-delivery timers (Erlang send_after).
+//
+// Arm a timer that delivers `msg` to an address after `after`, returning a
+// `TimerId`. The destination is resolved *on fire*, not at arm time: a
+// `Pid<A>` that has since died yields `SendError::Dead`, a `Name<M>` resolves
+// to whoever currently holds it (so a restarted server is reached). Either way
+// a failed resolve / closed inbox is dropped, matching `erlang:send_after`.
+// `cancel_timer` prevents an as-yet-unfired delivery; it returns whether the
+// timer was still armed.
+// ---------------------------------------------------------------------------
+
+/// Deliver `msg` to the exact actor named by `dest` (identity-bound, no
+/// redirect — see [`send_to`](crate::registry::send_to)) after `after`.
+/// Returns a [`TimerId`](crate::timer::TimerId) for [`cancel_timer`].
+pub fn send_after<A: crate::pid::Addressable>(
+    after: std::time::Duration,
+    dest: Pid<A>,
+    msg: A::Msg,
+) -> crate::timer::TimerId {
+    let deadline = crate::timer::deadline_from_now(after);
+    let fire = Box::new(move || {
+        let _ = crate::registry::send_to(dest, msg);
+    });
+    with_runtime(|inner| inner.timers.lock().unwrap().insert_send(deadline, dest.erase(), fire))
+}
+
+/// Deliver `msg` to whichever actor holds the name `dest` at fire time
+/// (re-resolving [`send`](crate::registry::send) semantics) after `after`.
+/// Returns a [`TimerId`](crate::timer::TimerId) for [`cancel_timer`].
+pub fn send_after_named<M: Send + 'static>(
+    after: std::time::Duration,
+    dest: Name<M>,
+    msg: M,
+) -> crate::timer::TimerId {
+    let deadline = crate::timer::deadline_from_now(after);
+    // Informational only (who armed it); not used for delivery.
+    let armed_by = current_pid().unwrap_or(Pid::new(0, 0));
+    let fire = Box::new(move || {
+        let _ = crate::registry::send(dest, msg);
+    });
+    with_runtime(|inner| inner.timers.lock().unwrap().insert_send(deadline, armed_by, fire))
+}
+
+/// Cancel a timer armed by [`send_after`] / [`send_after_named`]. Returns
+/// `true` if it was still pending (delivery now prevented), `false` if it had
+/// already fired or been cancelled.
+pub fn cancel_timer(id: crate::timer::TimerId) -> bool {
+    with_runtime(|inner| inner.timers.lock().unwrap().cancel(id))
+}
+
 // ---------------------------------------------------------------------------
 // block_on_io / wait_readable / wait_writable / read / write
 // ---------------------------------------------------------------------------