feat(domain): add MultiCoreScheduler with placement tracking, priority preemption, and load balancing

crates/domain/src/scheduler/per_core.rs

@@ -43,10 +43,15 @@ impl PerCoreScheduler {
         self.current = None;
     }
 
+    pub fn highest_waiting_priority(&self) -> Option<Priority> {
+        self.queue.highest_priority()
+    }
+
+    pub fn steal_lowest(&mut self) -> Option<(ThreadId, Priority)> {
+        self.queue.steal_lowest()
+    }
+
     pub fn len(&self) -> usize {
-        // for load balancing — count of ready threads
-        // could track this incrementally instead of computing
-        // but for now, bitmap popcount works
         self.queue.bitmap().count_ones() as usize
     }
 }