feat: add shared GMSK radio profile helpers

Add a fixed 6-byte 100 kbps GMSK profile matching the health-band telemetry packet, plus FSK packet-status helpers for RSSI and error checks. Expose instantaneous RSSI reads for CCA and image calibration for 430-440 MHz GFSK bring-up.

src/llcc68.cpp

@@ -569,6 +569,14 @@ expected<packet_status_t, error_code> LLCC68::get_packet_status() {
 	return ps;
 }
 
+expected<uint8_t, error_code> LLCC68::get_rssi_inst_x2_neg() {
+	uint8_t raw = 0;
+	auto r      = read_stream(RADIOLIB_SX126X_CMD_GET_RSSI_INST,
+							  std::span<uint8_t>{&raw, 1});
+	APP_RADIO_RETURN_ERR_IGNORE_PROC_ERR(r);
+	return raw;
+}
+
 expected<unit, error_code> LLCC68::reset() {
 	// Drive RESET low-high with delays per datasheet, then poll standby
 	if (not device_is_ready(config().reset_gpio.port)) {
@@ -899,6 +907,12 @@ expected<unit, error_code> LLCC68::set_gfsk_whitening_seed(uint16_t seed) {
 						  std::span<const uint8_t>{&lsb, 1});
 }
 
+expected<unit, error_code> LLCC68::calibrate_image(uint8_t freq1,
+												   uint8_t freq2) {
+	const uint8_t data[] = {freq1, freq2};
+	return write_stream(RADIOLIB_SX126X_CMD_CALIBRATE_IMAGE, data);
+}
+
 expected<unit, error_code> LLCC68::fix_inverted_iq(uint8_t iq_config) {
 	uint8_t iqCfg = 0;
 	auto r        = read_register(RADIOLIB_SX126X_REG_IQ_CONFIG,
@@ -1202,6 +1216,9 @@ LLCC68::hal_gfsk_modem_init(gfsk_parameters_t params) {
 
 	APP_RADIO_RETURN_ERR_CTX(set_packet_type_gfsk(),
 							 "gfsk_init::set_packet_type");
+	APP_RADIO_RETURN_ERR_CTX(calibrate_image(RADIOLIB_SX126X_CAL_IMG_430_MHZ_1,
+											 RADIOLIB_SX126X_CAL_IMG_430_MHZ_2),
+							 "gfsk_init::calibrate_image");
 	APP_RADIO_RETURN_ERR_CTX(set_gfsk_modulation_params(params.mod_params),
 							 "gfsk_init::set_modulation_params");
 	APP_RADIO_RETURN_ERR_CTX(