Improve persistence and reactor dynamics

src/reactor_sim/simulation.py

@@ -7,6 +7,7 @@ import json
 import logging
 import os
 from dataclasses import dataclass, field
+from pathlib import Path
 from threading import Event
 import time
 from typing import Callable, Iterable, Optional
@@ -21,6 +22,11 @@ LOGGER = logging.getLogger(__name__)
 CommandProvider = Callable[[float, PlantState], Optional[ReactorCommand]]
 
 
+def _default_state_path() -> Path:
+    custom = os.getenv("FISSION_STATE_PATH")
+    return Path(custom) if custom else Path("artifacts/last_state.json")
+
+
 @dataclass
 class ReactorSimulation:
     reactor: Reactor
@@ -80,8 +86,11 @@ def main() -> None:
         timestep = 1.0 if dashboard_mode or realtime else 5.0
 
     sim = ReactorSimulation(reactor, timestep=timestep, duration=duration, realtime=realtime)
+    state_path = _default_state_path()
     load_path = os.getenv("FISSION_LOAD_STATE")
-    save_path = os.getenv("FISSION_SAVE_STATE")
+    if not load_path and state_path.exists():
+        load_path = str(state_path)
+    save_path = os.getenv("FISSION_SAVE_STATE") or str(state_path)
     if load_path:
         sim.start_state = reactor.load_state(load_path)
     if dashboard_mode: