LocalNoiseFloor

The connected device's own most recently reported noise floor, in dBm (from its LocalStats telemetry), provided once at the app root (see MeshtasticAppShell) so signal quality can be rated against it without threading it through every node/message composable. This is always our receiving radio's noise floor — the connected node is the one that heard any remote node's packets being rated, regardless of which remote node's signal is displayed.

Null before a device connects, in previews/tests, and when the connected node has not yet reported a reading (LocalStats.noise_floor == 0, the firmware's "no reading yet" sentinel — normalized to null at the source so callers never need to know about it).