feat: add reactor control persistence and tests

tests/test_simulation.py

@@ -0,0 +1,42 @@
+import json
+from pathlib import Path
+
+import pytest
+
+from reactor_sim import constants
+from reactor_sim.reactor import Reactor
+from reactor_sim.simulation import ReactorSimulation
+
+
+def test_reactor_initial_state_is_cold():
+    reactor = Reactor.default()
+    state = reactor.initial_state()
+    assert state.core.fuel_temperature == constants.ENVIRONMENT_TEMPERATURE
+    assert state.primary_loop.mass_flow_rate == 0.0
+    assert state.turbine.electrical_output_mw == 0.0
+
+
+def test_state_save_and_load_roundtrip(tmp_path: Path):
+    reactor = Reactor.default()
+    sim = ReactorSimulation(reactor, timestep=5.0, duration=15.0)
+    sim.log()
+    save_path = tmp_path / "plant_state.json"
+    assert sim.last_state is not None
+    reactor.save_state(str(save_path), sim.last_state)
+    assert save_path.exists()
+    restored_reactor = Reactor.default()
+    restored_state = restored_reactor.load_state(str(save_path))
+    assert restored_state.core.fuel_temperature == pytest.approx(
+        sim.last_state.core.fuel_temperature
+    )
+    assert restored_reactor.control.rod_fraction == reactor.control.rod_fraction
+
+
+def test_health_monitor_flags_core_failure():
+    reactor = Reactor.default()
+    state = reactor.initial_state()
+    state.core.fuel_temperature = constants.MAX_CORE_TEMPERATURE
+    failures = reactor.health_monitor.evaluate(state, True, True, True, dt=200.0)
+    assert "core" in failures
+    reactor._handle_failure("core")
+    assert reactor.shutdown is True