snrOrNull

Last measured SNR in dB, or null when this node has no reading yet (snr still holds SNR_UNSET).

Every read of snr should go through this: 0 dB is a real, good reading, and the raw sentinel rates as an excellent signal if it reaches the preset-relative quality bands. Threshold comparisons such as snr < 100f are not equivalent — they also discard any genuine reading at or above the threshold.