RFC 016 Chunk 2b: per-actor messages-received counter

Tally each dequeued message against the receiving actor, surfaced as
ActorInfo.messages_received (the 'is this actor draining slower than its
mailbox fills' signal).

- messages-received, not sent (D4): the receiver counts on its own
  thread, so it's a single-writer Relaxed load+store on a hot Slot
  AtomicU64, no atomic RMW (D5); reuses the stashed *const Slot from 2a.
- Incremented at all six channel dequeue-success sites (recv,
  recv_timeout x2, recv_match, try_recv_match, try_recv) via
  preempt::note_message_received; no-op outside an actor (null slot).
- Resets with overruns in reset_counters across the three lifecycle
  sites (D7).

Matrix: debug default + rq-mpmc + rq-striped + release green; loom
slot_state/run_queue models pass (the 3 send_after_to loom failures are
pre-existing at fc014c4 — plain #[test]s under --cfg loom, not Chunk 2).

src/channel.rs

@@ -170,6 +170,7 @@ impl<T> Receiver<T> {
             {
                 let mut g = self.inner.lock();
                 if let Some(v) = g.queue.pop_front() {
+                    crate::preempt::note_message_received();
                     return Ok(v);
                 }
                 if g.senders == 0 {
@@ -229,6 +230,7 @@ impl<T> Receiver<T> {
         {
             let mut g = self.inner.lock();
             if let Some(v) = g.queue.pop_front() {
+                crate::preempt::note_message_received();
                 return Ok(v);
             }
             if g.senders == 0 {
@@ -255,6 +257,7 @@ impl<T> Receiver<T> {
         crate::te!(crate::trace::Event::RecvWake(crate::actor::current_pid().unwrap()));
         let mut g = self.inner.lock();
         if let Some(v) = g.queue.pop_front() {
+            crate::preempt::note_message_received();
             return Ok(v);
         }
         if g.senders == 0 {
@@ -282,6 +285,7 @@ impl<T> Receiver<T> {
                 let mut g = self.inner.lock();
                 if let Some(i) = g.queue.iter().position(|v| pred(v)) {
                     // position() found it, so remove() returns Some.
+                    crate::preempt::note_message_received();
                     return Ok(g.queue.remove(i).unwrap());
                 }
                 if g.senders == 0 {
@@ -313,6 +317,7 @@ impl<T> Receiver<T> {
     {
         let mut g = self.inner.lock();
         if let Some(i) = g.queue.iter().position(|v| pred(v)) {
+            crate::preempt::note_message_received();
             return Ok(Some(g.queue.remove(i).unwrap()));
         }
         if g.senders == 0 {
@@ -326,6 +331,7 @@ impl<T> Receiver<T> {
     pub fn try_recv(&self) -> Result<Option<T>, RecvError> {
         let mut g = self.inner.lock();
         if let Some(v) = g.queue.pop_front() {
+            crate::preempt::note_message_received();
             return Ok(Some(v));
         }
         if g.senders == 0 {