Bias shutdown reactivity and keep cold start subcritical

src/reactor_sim/reactor.py

@@ -79,6 +79,7 @@ class Reactor:
         # Default to a cold, safe configuration: rods fully inserted, manual control, pumps/turbines off.
         self.control.manual_control = True
         self.control.rod_fraction = 0.95
+        self.shutdown = True
         self.primary_pump_active = False
         self.secondary_pump_active = False
         self.turbine_unit_active = [False] * len(self.turbines)