Add boron reactivity trim and power measurement smoothing

CONTEXT_NOTES.md

@@ -3,6 +3,7 @@
 - Reactor model: two-loop but tuned to RBMK-like pressures (nominal ~7 MPa for both loops). Loop pressure clamps to saturation baseline when pumps are off; pumps ramp flow/pressure over spool time when stopping or starting.
 - Feedwater & level: secondary steam drum uses shrink/swell-aware level sensing to drive a feedwater valve; makeup flow scales with steam draw toward the target level instead of instant inventory clamps.
 - Chemistry/fouling: plant tracks dissolved O2/boron/sodium; impurities plus high temp/steam draw increase HX fouling (reduces UA) and add condenser fouling/back-pressure. Oxygen degasses with steam; impurity ingress accelerates when venting.
+- Reactivity bias: boron ppm now biases shutdown reactivity; a very slow trim nudges boron toward the power setpoint after ~300s of operation.
 - Turbines: produce zero output unless steam quality is present and effective steam flow is >10 kg/s. Turbine panel shows steam availability (enthalpy × quality × mass flow) and steam enthalpy instead of loop pressure; condenser pressure/temperature/fouling shown with nominal bounds.
 - Generators: two diesel units, rated 50 MW, spool time 10s. Auto mode default `False`; manual toggles b/v. Auto stops when no load. Relief valves toggles l (primary) / ; (secondary) and displayed per loop.
 - Pumps: per-unit controls g/h (primary), j/k (secondary). Flow/pressure ramp down over spool when pumps stop. Pump status thresholds use >0.1 kg/s to show STOPPING.