Without period, y=k*t+b and y=base+drift_rate*t grow infinitely, which is
unrealistic for industrial simulation. period_ms makes t wrap modulo the period,
producing sawtooth patterns that simulate per-cycle behavior (e.g. tension build-up
per coil, sensor drift per shift).
Add CompositeModel for combining base+noise models, ModelRegistry singleton
with JSON-based model template loading, per-instance-key model isolation,
and inline CSV/valve pair/composite syntax parsing in createModel.
