Interface SpectrumTier

A LOW-tier spectrum, riding alongside a SpectrumMessage's HIGH-tier top-level fields (issue #95, "zoom-FFT"): same shape, same fftSize, a ~8× lower sampleRate — see mix_dsp.h's decimated LOW tier.

interface SpectrumTier {
    fftSize: number;
    mags: readonly number[];
    sampleRate: number;
}