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.

src/introspect.rs

@@ -83,6 +83,10 @@ pub struct ActorInfo {
     /// install / spawn_addr / gen_server). 0 for an actor that holds only a
     /// private `channel()` receiver — those are invisible to the registry.
     pub mailbox_depth: u32,
+    /// Timeslice overruns tallied for this incarnation (RFC 016 Chunk 2): how
+    /// many times the actor was preempted for exceeding its slice. Resets on
+    /// restart (per-incarnation, D7).
+    pub overruns: u64,
 }
 
 /// A whole-runtime snapshot. See the module docs for the D2 tearing model.
@@ -159,6 +163,9 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
     let joiners = cold.waiters.len() as u32;
     drop(cold);
 
+    // Counters are hot-region atomics, read lock-free (RFC 016 Chunk 2).
+    let overruns = slot.overruns();
+
     // Names + depth belong to this incarnation only if the registry mailbox's
     // pid matches the slab generation; a stale registry entry (dead prior
     // occupant, not yet pruned) contributes nothing.
@@ -177,6 +184,7 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
         links,
         joiners,
         mailbox_depth,
+        overruns,
     })
 }