Improve persistence and reactor dynamics

src/reactor_sim/neutronics.py

@@ -15,12 +15,12 @@ LOGGER = logging.getLogger(__name__)
 def temperature_feedback(temp: float) -> float:
     """Negative coefficient: higher temperature lowers reactivity."""
     reference = 900.0
-    coefficient = -5e-5
+    coefficient = -2.5e-5
     return coefficient * (temp - reference)
 
 
 def xenon_poisoning(flux: float) -> float:
-    return min(0.05, 1e-8 * flux)
+    return min(0.015, 2e-9 * flux)
 
 
 @dataclass