Four if/elif ladders (SNR and RSSI, for DMs and channel messages) checked a list of payload keys, then a list of metadata keys, converting the first present value. _signal_value takes the key lists and converter, so each site keeps its own keys: DMs still accept signal_to_noise and signal_noise_ratio, channel messages do not. A present-but-None key still ends the search, metadata is still read only when no payload key is present, and the cache and RF fallbacks after each call are untouched. A test checks it against the original ladder for every combination of absent, None and valued keys.
