feat: GetMac command + MacAddress response

New protocol command (tag 8): GetMac returns the board's 6-byte hardware
address. Useful when multiple dongles are connected to one host.

Response (tag 6): MacAddress containing 6 raw bytes.

Updated: PROTOCOL.md, CHANGELOG, donglora.py decoder, all_commands.py.

src/radio/task.rs

@@ -219,7 +219,7 @@ async fn handle_cmd(
             status.sender().send(state.clone());
             responses.send(Response::Ok).await;
         }
-        Command::DisplayOn | Command::DisplayOff => {}
+        Command::DisplayOn | Command::DisplayOff | Command::GetMac => {}
         Command::Transmit { config, payload } => {
             let tx_config = config.map(|c| c.resolve_power(Board::TX_POWER_RANGE)).or(state.config);
             if let Some(cfg) = tx_config {