Add pressurizer and coolant inventory controls

src/reactor_sim/state.py

@@ -43,6 +43,8 @@ class CoolantLoopState:
     pressure: float  # MPa
     mass_flow_rate: float  # kg/s
     steam_quality: float  # fraction of vapor
+    inventory_kg: float = 0.0  # bulk mass of coolant
+    level: float = 1.0  # fraction full relative to nominal volume
 
     def average_temperature(self) -> float:
         return 0.5 * (self.temperature_in + self.temperature_out)