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/timer.rs

@@ -15,12 +15,14 @@
 //! `BinaryHeap` is a max-heap; entries are wrapped in `Reverse` to get
 //! min-heap behaviour.
 //!
-//! No cancellation. When a non-timer wakeup happens (e.g. lock granted
-//! before timeout), the timer entry is left in the heap. It will be popped
-//! eventually and the dispatch will observe "actor is no longer parked /
-//! the wait's epoch was consumed" and no-op. Cost is ~32 bytes per stale
-//! entry plus a few cycles on pop; acceptable given the upper bound is "one
-//! entry per parked actor".
+//! Cancellation is selective. A `Sleep` / `WaitTimeout` entry is left in the
+//! heap on a non-timer wakeup (lock granted before timeout): it is popped
+//! eventually and no-ops because a stale unpark fails its epoch CAS — cheap
+//! (~32 bytes per stale entry plus a few cycles on pop), bounded by one entry
+//! per parked actor. A `Send` entry is different: running its thunk delivers a
+//! real message, so a stale one is *not* inert. `send_after` therefore carries
+//! true cancellation via the `armed` set keyed on the entry's `seq`; `pop_due`
+//! fires a `Send` only while it is still armed, and `cancel` removes the arm.
 //!
 //! Stale pids (slot reused since the timer was inserted) are filtered on
 //! pop by the scheduler — same convention as the run queue.
@@ -51,8 +53,28 @@ pub enum Reason {
         target: Arc<dyn TimerTarget>,
         epoch: u32,
     },
+    /// `send_after`: deliver a message to an address at the deadline,
+    /// cancellable. The destination (a `Pid<A>` / `Name<M>`) and the message
+    /// are captured inside `fire`, which resolves the address through the
+    /// registry and sends *when run* — so a target that died or, for a name,
+    /// was restarted is observed at fire time, not arm time. A failed resolve
+    /// or send is dropped (Erlang `erlang:send_after` semantics).
+    ///
+    /// Unlike `Sleep` / `WaitTimeout`, a stale `Send` is **not** inert — running
+    /// the thunk delivers a real message — so these are the only timers that
+    /// carry true cancellation (the `armed` set on [`Timers`], keyed by the
+    /// entry's `seq`). `pop_due` fires the thunk only for an entry still armed.
+    Send { fire: Box<dyn FnOnce() + Send> },
 }
 
+/// Opaque handle to an armed `send_after` timer, returned by
+/// [`Timers::insert_send`] and consumed by [`Timers::cancel`]. The inner value
+/// is the entry's insertion `seq`; callers must treat it as opaque so the
+/// backing structure can change (e.g. a future hierarchical timing wheel) with
+/// no API churn.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct TimerId(u64);
+
 /// Callback the scheduler invokes when a `WaitTimeout` entry pops.
 ///
 /// Implementors: do not touch `SchedulerState` other than via the public
@@ -97,13 +119,21 @@ impl PartialOrd for Entry {
 pub struct Timers {
     /// Reverse-wrapped so the smallest deadline is at the top.
     heap: BinaryHeap<Reverse<Entry>>,
-    /// Monotonic counter for the tiebreaker `seq` field.
+    /// Monotonic counter for the tiebreaker `seq` field (and the `TimerId` of a
+    /// `Send` timer — the two are the same value).
     next_seq: u64,
+    /// Presence set of *live* `Send` timers, keyed by `seq`. Populated on
+    /// `insert_send`, removed on fire (in `pop_due`) and on `cancel`. A `Send`
+    /// entry fires only while present, so a `cancel` that lands before the
+    /// entry pops prevents delivery; a `cancel` after it has fired finds
+    /// nothing (the race signal). Bounded by armed-but-not-yet-resolved timers
+    /// and self-collecting — no sweep. `Sleep` / `WaitTimeout` never touch it.
+    armed: std::collections::HashSet<u64>,
 }
 
 impl Timers {
     pub fn new() -> Self {
-        Self { heap: BinaryHeap::new(), next_seq: 0 }
+        Self { heap: BinaryHeap::new(), next_seq: 0, armed: std::collections::HashSet::new() }
     }
 
     /// Insert a `Sleep` timer. Convenience for the common case.
@@ -111,6 +141,33 @@ impl Timers {
         self.insert(deadline, pid, Reason::Sleep { epoch });
     }
 
+    /// Arm a cancellable `send_after` timer: run `fire` at `deadline` unless
+    /// [`cancel`](Self::cancel)led first. `pid` is informational only (the
+    /// destination, or who armed it — useful for introspection); it is *not*
+    /// used to wake anyone, the delivery lives entirely inside `fire`. Returns
+    /// a [`TimerId`] for cancellation.
+    pub fn insert_send(
+        &mut self,
+        deadline: Instant,
+        pid: Pid,
+        fire: Box<dyn FnOnce() + Send>,
+    ) -> TimerId {
+        let seq = self.next_seq;
+        self.next_seq = self.next_seq.wrapping_add(1);
+        self.armed.insert(seq);
+        self.heap.push(Reverse(Entry { deadline, seq, pid, reason: Reason::Send { fire } }));
+        TimerId(seq)
+    }
+
+    /// Cancel an armed `send_after` timer. Returns `true` if the timer was
+    /// still armed (delivery is now prevented), `false` if it had already
+    /// fired or been cancelled. The heap entry, if still pending, is left to be
+    /// discarded when its deadline passes — `pop_due` drops any `Send` entry
+    /// whose `seq` is no longer armed.
+    pub fn cancel(&mut self, id: TimerId) -> bool {
+        self.armed.remove(&id.0)
+    }
+
     /// Insert an arbitrary timer entry.
     pub fn insert(&mut self, deadline: Instant, pid: Pid, reason: Reason) {
         let seq = self.next_seq;
@@ -127,6 +184,7 @@ impl Timers {
     /// discarded so it can't keep the runtime alive.
     pub fn clear(&mut self) {
         self.heap.clear();
+        self.armed.clear();
     }
 
     /// Soonest pending deadline, or `None` if the heap is empty.
@@ -136,11 +194,21 @@ impl Timers {
 
     /// Pop every entry whose deadline is ≤ `now`, in deadline order.
     /// The scheduler dispatches each entry by inspecting `entry.reason`.
+    ///
+    /// A due `Send` entry is returned only if it is still armed; a cancelled
+    /// one is silently dropped here (its `seq` was already removed from
+    /// `armed` by [`cancel`](Self::cancel)). Returning it removes it from
+    /// `armed`, so a later `cancel` of a fired timer reports `false`.
     pub fn pop_due(&mut self, now: Instant) -> Vec<Entry> {
         let mut out = Vec::new();
         while let Some(r) = self.heap.peek() {
             if r.0.deadline <= now {
-                out.push(self.heap.pop().unwrap().0);
+                let entry = self.heap.pop().unwrap().0;
+                if matches!(entry.reason, Reason::Send { .. }) && !self.armed.remove(&entry.seq) {
+                    // Cancelled before it came due: discard, do not deliver.
+                    continue;
+                }
+                out.push(entry);
             } else {
                 break;
             }