Expose xenon reactivity impact on dashboard

tests/test_neutronics.py

@@ -1,3 +1,5 @@
+import pytest
+
 from reactor_sim.neutronics import NeutronDynamics
 from reactor_sim.state import CoreState
 
@@ -32,3 +34,12 @@ def test_poisons_accumulate_under_power():
     dynamics.update_poisons(state, dt=100.0)
     assert state.iodine_inventory > 0.0
     assert state.xenon_inventory > 0.0
+
+
+def test_xenon_penalty_caps():
+    dynamics = NeutronDynamics()
+    state = _core_state()
+    state.xenon_inventory = 50.0
+    assert dynamics.xenon_penalty(state) == 0.03
+    state.xenon_inventory = 5.0
+    assert dynamics.xenon_penalty(state) == pytest.approx(0.01)