fix(runtime): stale-PID pop retries immediately instead of taking the idle path

The phase-1 split filled Idle::next_deadline from the timers lock after the
shared lock was released, which turned the stale-PID branch (formerly a 100µs
poll) into a potential sleep-until-timer-deadline while runnable actors sat in
the queue — a hard stall on exact(1). Stale pops are now PopResult::Retry and
loop without sleeping.

test(poison): strengthen regression coverage. The stop-storm test never fired
the sentinel under a lock (its actors only allocate at lock-free points). Add
self_stop_during_spawn_does_not_poison_shared_mutex: a stop-flagged actor whose
next allocation is the Box::new(closure) inside spawn's with_shared. Validated
both ways: passes with the gate, SIGABRTs (unwind-in-allocator) with the gate
removed. Also alloc_interval(1) so every allocation is an observation point.

tests/poison_stop.rs

@@ -10,7 +10,10 @@ use std::sync::Arc;
 
 #[test]
 fn stop_storm_does_not_poison_runtime() {
-    let rt = init(Config::exact(4));
+    // alloc_interval(1): every allocation is an observation point, so pre-fix
+    // the sentinel fires inside with_shared's alloc sites (run_queue.push_back,
+    // waiters.push, ...) with near-certainty rather than 1-in-128.
+    let rt = init(Config::exact(4).alloc_interval(1));
     let completed = Arc::new(AtomicUsize::new(0));
     let c = completed.clone();
     rt.run(move || {
@@ -40,3 +43,47 @@ fn stop_storm_does_not_poison_runtime() {
     });
     assert_eq!(completed.load(Ordering::SeqCst), 1, "root completed cleanly");
 }
+
+/// The sharper repro: a stop-flagged actor whose *next allocation* is the
+/// `Box::new(closure)` inside `spawn`'s `with_shared` critical section.
+/// Pre-fix, the ungated `check_cancelled` in `maybe_preempt` raises the
+/// sentinel right there, unwinding while the global shared mutex is held →
+/// poisoned → every later `with_shared` panics on every scheduler thread and
+/// the whole runtime collapses. Post-fix, observation is deferred to the next
+/// lock-free point and each actor dies a clean `Outcome::Stopped`.
+///
+/// `request_stop(self_pid())` sets the flag without parking (we're running),
+/// and the capturing closure makes `Box::new(f)` a real allocation. With
+/// `alloc_interval(1)` the check fires every other allocation, so each actor
+/// has ~50% odds of the check landing on the under-lock alloc; 64 actors make
+/// a pre-fix escape astronomically unlikely.
+#[test]
+fn self_stop_during_spawn_does_not_poison_shared_mutex() {
+    let rt = init(Config::exact(4).alloc_interval(1));
+    let completed = Arc::new(AtomicUsize::new(0));
+    let c = completed.clone();
+    rt.run(move || {
+        let mut handles = Vec::new();
+        for i in 0..64usize {
+            let h = smarm::spawn(move || {
+                // Vary pre-stop allocation count to cover both phases of the
+                // every-other-allocation check cadence.
+                for _ in 0..(i % 2) {
+                    let _phase: Vec<u8> = Vec::with_capacity(8);
+                }
+                let payload = vec![0u8; 64]; // captured → Box::new(f) allocates
+                smarm::request_stop(smarm::self_pid());
+                let grandchild = smarm::spawn(move || drop(payload));
+                let _ = grandchild.join();
+            });
+            handles.push(h);
+        }
+        for h in handles {
+            // Stopped reports Ok from join; the point is that join itself
+            // (with_shared) doesn't panic on a poisoned mutex.
+            let _ = h.join();
+        }
+        c.fetch_add(1, Ordering::SeqCst);
+    });
+    assert_eq!(completed.load(Ordering::SeqCst), 1, "root completed cleanly");
+}