Add rod banks, xenon kinetics, and document feature set

FEATURES.md

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+## C.O.R.E. feature set
+
+- **Core physics**: point-kinetics with delayed neutrons, temperature feedback, fuel burnup penalty, xenon/iodine buildup with decay and burn-out, and rod-bank worth curves.
+- **Rod control**: three rod banks with weighted worth; auto controller chases 3 GW setpoint; manual mode with staged bank motion and SCRAM; state persists across runs.
+- **Coolant & hydraulics**: primary/secondary pumps with head/flow curves, power draw scaling, wear tracking; pressure floors tied to saturation; auxiliary power model with generator auto-start.
+- **Heat transfer**: steam-generator UA·ΔT_lm model; core and secondary thermal solvers with passive cool-down when flow is low.
+- **Steam cycle**: three turbines with spool dynamics, load dispatch to consumer, steam quality gating for output, generator states with batteries/spool.
+- **Protections & failures**: health monitor degrading components under stress, automatic SCRAM on core or heat-sink loss, relief valves per loop, maintenance actions to restore integrity.
+- **Persistence & ops**: snapshots auto-save/load to `artifacts/last_state.json`; dashboard with live metrics, protections/warnings, heat-exchanger telemetry, component health, and control shortcuts.