@spec parse_callsign(String.t()) ::
  {:ok, {String.t(), String.t()}} | {:error, atom() | String.t()}

Parse and validate an AX.25 callsign.

Returns {:ok, {base_callsign, ssid}}, with an SSID of "0" when the callsign carries none, or {:error, reason} when the callsign is not a legal AX.25 address.