RFC 016 Chunk 2a: per-actor timeslice overrun counter

Tally overruns at the slice-expiry site in preempt.rs (the RFC 006
signal), surfaced as ActorInfo.overruns.

- Counter is a hot-region AtomicU64 on Slot, single-writer Relaxed
  load+store (no atomic RMW), read Relaxed by the snapshot (D5).
- Reached from the rare expiry branch via a stashed *const Slot in a
  preempt thread-local, set/cleared on the same resume/return boundary
  as CURRENT_STOP — one TLS load, no runtime lookup. Shared infra for
  the messages-received counter next.
- Reset in all three slot-lifecycle sites: Slot::vacant, reclaim_slot,
  install_actor (standing invariant, D7) — per-incarnation counts.

tests/introspect.rs

@@ -234,6 +234,7 @@ fn tree_from_nests_children_and_reroots_orphans() {
         links: 0,
         joiners: 0,
         mailbox_depth: 0,
+        overruns: 0,
     };
 
     let snap = RuntimeSnapshot {
@@ -258,3 +259,30 @@ fn tree_from_nests_children_and_reroots_orphans() {
     assert!(o.orphaned, "an actor whose parent is absent must be flagged orphaned");
     assert!(o.children.is_empty());
 }
+
+#[test]
+fn overrun_count_increments_on_forced_preemption() {
+    run(|| {
+        // A worker that forces its slice to expire, then hits an observation
+        // point so the slice-expiry site fires and tallies one overrun.
+        let (ready_tx, ready_rx) = channel::<()>();
+        let (gate_tx, gate_rx) = channel::<()>();
+        let h = spawn(move || {
+            smarm::preempt::expire_timeslice_for_test();
+            smarm::check!(); // preempt-yield here → one overrun tallied
+            ready_tx.send(()).unwrap();
+            gate_rx.recv().unwrap();
+        });
+        ready_rx.recv().unwrap(); // worker is past the forced preemption
+
+        let info = actor_info(h.pid()).expect("worker present");
+        assert!(
+            info.overruns >= 1,
+            "forced timeslice expiry should tally at least one overrun, got {}",
+            info.overruns
+        );
+
+        gate_tx.send(()).unwrap();
+        h.join().unwrap();
+    });
+}