Add cooldown telemetry, RHR cooldown path, and shutdown regression test

tests/test_simulation.py

@@ -341,3 +341,38 @@ def test_full_power_reaches_steam_and_turbine_output():
     assert results[3600]["electric"] > 150.0
     # No runaway core temperatures.
     assert results[3600]["core_temp"] < constants.CORE_MELTDOWN_TEMPERATURE
+
+
+def test_cooldown_reaches_ambient_without_runaway():
+    """Shutdown with pumps running should cool loops toward ambient, no runaway."""
+    reactor = Reactor.default()
+    reactor.health_monitor.disable_degradation = True
+    reactor.allow_external_aux = True
+    reactor.relaxed_npsh = True
+    state = reactor.initial_state()
+    # Start hot.
+    reactor.step(
+        state,
+        dt=1.0,
+        command=ReactorCommand(
+            generator_units={1: True, 2: True},
+            primary_pumps={1: True, 2: True},
+            secondary_pumps={1: True, 2: True},
+            rod_manual=True,
+            rod_position=0.55,
+        ),
+    )
+    turbines_started = False
+    for i in range(1800):
+        cmd = None
+        if not turbines_started and state.secondary_loop.steam_quality > 0.02 and state.secondary_loop.pressure > 1.0:
+            cmd = ReactorCommand(turbine_on=True, turbine_units={1: True, 2: True, 3: True})
+            turbines_started = True
+        if i == 900:
+            cmd = ReactorCommand(rod_position=0.95, turbine_on=False, turbine_units={1: False, 2: False, 3: False})
+        reactor.step(state, dt=1.0, command=cmd)
+
+    assert not reactor.meltdown
+    assert state.core.power_output_mw < 1.0
+    assert state.primary_loop.temperature_out < 320.0
+    assert state.secondary_loop.temperature_out < 320.0