Improve persistence and reactor dynamics

src/reactor_sim/state.py

@@ -49,7 +49,7 @@ class PlantState:
     core: CoreState
     primary_loop: CoolantLoopState
     secondary_loop: CoolantLoopState
-    turbine: TurbineState
+    turbines: list[TurbineState]
     time_elapsed: float = field(default=0.0)
 
     def snapshot(self) -> dict[str, float]:
@@ -60,18 +60,27 @@ class PlantState:
             "neutron_flux": self.core.neutron_flux,
             "primary_outlet_temp": self.primary_loop.temperature_out,
             "secondary_pressure": self.secondary_loop.pressure,
-            "turbine_electric": self.turbine.electrical_output_mw,
+            "turbine_electric": self.total_electrical_output(),
         }
 
+    def total_electrical_output(self) -> float:
+        return sum(t.electrical_output_mw for t in self.turbines)
+
     def to_dict(self) -> dict:
         return asdict(self)
 
     @classmethod
     def from_dict(cls, data: dict) -> "PlantState":
+        turbines_blob = data.get("turbines")
+        if turbines_blob is None:
+            # Compatibility with previous single-turbine snapshots.
+            old_turbine = data.get("turbine")
+            turbines_blob = [old_turbine] if old_turbine else []
+        turbines = [TurbineState(**t) for t in turbines_blob]
         return cls(
             core=CoreState(**data["core"]),
             primary_loop=CoolantLoopState(**data["primary_loop"]),
             secondary_loop=CoolantLoopState(**data["secondary_loop"]),
-            turbine=TurbineState(**data["turbine"]),
+            turbines=turbines,
             time_elapsed=data.get("time_elapsed", 0.0),
         )