Add rod banks, xenon kinetics, and document feature set

src/reactor_sim/state.py

@@ -18,6 +18,8 @@ class CoreState:
     reactivity_margin: float  # delta rho
     power_output_mw: float  # MW thermal
     burnup: float  # fraction of fuel consumed
+    xenon_inventory: float = 0.0
+    iodine_inventory: float = 0.0
     fission_product_inventory: dict[str, float] = field(default_factory=dict)
     emitted_particles: dict[str, float] = field(default_factory=dict)