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/timer.rs

@@ -119,6 +119,13 @@ impl Timers {
         self.heap.is_empty()
     }
 
+    /// Drop all pending entries. Called by the scheduler when it has decided
+    /// no actor is live: any remaining timer is orphaned and exists only to be
+    /// discarded so it can't keep the runtime alive.
+    pub fn clear(&mut self) {
+        self.heap.clear();
+    }
+
     /// Soonest pending deadline, or `None` if the heap is empty.
     pub fn peek_deadline(&self) -> Option<Instant> {
         self.heap.peek().map(|r| r.0.deadline)