Add DNB/subcooling margins and keypad rod control

src/reactor_sim/reactor.py

@@ -402,6 +402,12 @@ class Reactor:
 
         if (not self.secondary_pump_active or state.secondary_loop.mass_flow_rate <= 1.0) and total_power > 50.0:
             self._handle_heat_sink_loss(state)
+        if state.core.dnb_margin is not None and state.core.dnb_margin < 0.3:
+            LOGGER.critical("DNB margin low: %.2f, initiating SCRAM", state.core.dnb_margin)
+            self.shutdown = True
+            self.control.scram()
+        if state.core.subcooling_margin is not None and state.core.subcooling_margin < 5.0:
+            LOGGER.warning("Subcooling margin low: %.1fK", state.core.subcooling_margin)
 
         failures = self.health_monitor.evaluate(
             state,