Function infer_protocol 

fn infer_protocol(callsign: &str) -> &'static str

Infers the D-STAR protocol from a reflector callsign prefix.

• REF prefix → "dplus"
• XRF or XLX prefix → "dextra"
• DCS prefix → "dcs"
• Anything else → "dextra" (conservative default for unknown reflectors)