fix(runtime): don't let orphaned timers block shutdown

A cooperatively-cancelled actor that was sleeping (or in a bounded wait)
leaves its timer entry behind in the heap. The scheduler's shutdown
check required "no pending timers", so a single orphaned far-future
deadline kept the whole runtime alive until it fired — e.g. cancelling
an actor mid sleep(30s) hung run() for 30s.

A timer can only ever do useful work by waking a live actor, so once no
actors are live every remaining entry is orphaned by definition. Drop
the pending-timers condition from the shutdown check (live == 0 already
implies nothing a timer could wake) and clear the heap on the way out.

src/runtime.rs

@@ -779,11 +779,17 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot: usize) {
                 }
             } else {
                 // Queue was empty. Re-examine to decide exit vs wait.
-                // All four conditions must hold simultaneously before we exit:
+                // We exit only when nothing can ever produce more work:
                 //   1. run queue is still empty
                 //   2. no live actors (nothing parked, nothing mid-finalize)
-                //   3. no pending timers
-                //   4. no outstanding IO
+                //   3. no outstanding IO
+                // Pending timers are deliberately NOT a condition: a timer can
+                // only ever do something useful by waking a live actor, so once
+                // `live == 0` every remaining timer entry is orphaned (e.g. a
+                // sleeping actor that was cooperatively cancelled out of its
+                // sleep, leaving its deadline behind). Such entries must not
+                // keep the runtime alive until they fire — that could be an
+                // arbitrarily long hang. They are dropped here on the way out.
                 let next = s.timers.peek_deadline();
                 let (out, fd) = match s.io.as_ref() {
                     Some(io) => (
@@ -793,9 +799,9 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot: usize) {
                     None => (0, None),
                 };
                 let live = s.slots.iter().filter(|slot| slot.actor.is_some()).count();
-                let all_clear = s.run_queue.is_empty() && live == 0
-                    && next.is_none() && out == 0;
+                let all_clear = s.run_queue.is_empty() && live == 0 && out == 0;
                 if all_clear {
+                    s.timers.clear();
                     PopResult::AllDone
                 } else {
                     PopResult::Idle { next_deadline: next, io_outstanding: out, wake_fd: fd }