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/introspect.rs

@@ -87,6 +87,10 @@ pub struct ActorInfo {
     /// many times the actor was preempted for exceeding its slice. Resets on
     /// restart (per-incarnation, D7).
     pub overruns: u64,
+    /// Messages this actor has received (dequeued) this incarnation (RFC 016
+    /// Chunk 2) — answers "is this actor a hotspot / draining slower than its
+    /// mailbox fills." Counts received, not sent (D4). Per-incarnation (D7).
+    pub messages_received: u64,
 }
 
 /// A whole-runtime snapshot. See the module docs for the D2 tearing model.
@@ -165,6 +169,7 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
 
     // Counters are hot-region atomics, read lock-free (RFC 016 Chunk 2).
     let overruns = slot.overruns();
+    let messages_received = slot.messages_received();
 
     // Names + depth belong to this incarnation only if the registry mailbox's
     // pid matches the slab generation; a stale registry entry (dead prior
@@ -185,6 +190,7 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
         joiners,
         mailbox_depth,
         overruns,
+        messages_received,
     })
 }