Interface StatefulLensSpec1<S, V, C>

Single-source stateful spec — the scalar fast-path of StatefulLensSpec: one parent, so init/step/fwd/bwd take the source value directly, not a tuple.

interface StatefulLensSpec1<S, V, C> {
    bwd: (target: V, source: S, complement: C) => StatefulBwd1<S, C>;
    fwd: (source: S, complement: C) => V;
    init: (source: S) => C;
    step?: (source: S, complement: C) => C;
}