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.

include/llcc68.hpp

@@ -125,6 +125,7 @@ struct LLCC68 {
 
 	expected<status_t, error_code> get_status();
 	expected<packet_status_t, error_code> get_packet_status();
+	expected<uint8_t, error_code> get_rssi_inst_x2_neg();
 	expected<unit, error_code> reset();
 	expected<unit, error_code> set_standby();
 	expected<ChipType, error_code> hal_get_chip_type();
@@ -164,6 +165,7 @@ struct LLCC68 {
 	expected<unit, error_code> set_gfsk_crc_seed(uint16_t seed);
 	expected<unit, error_code> set_gfsk_crc_polynomial(uint16_t polynomial);
 	expected<unit, error_code> set_gfsk_whitening_seed(uint16_t seed);
+	expected<unit, error_code> calibrate_image(uint8_t freq1, uint8_t freq2);
 	expected<unit, error_code> set_tx_params(tx_params_t params);
 	expected<unit, error_code> set_tx_params(int8_t pwr,
 											 LoRaTxRampTime ramp_time);

include/llcc68_definitions.hpp

@@ -476,6 +476,40 @@ struct gfsk_parameters_t {
 			.broadcast_address = std::nullopt,
 		};
 	}
+
+	static constexpr gfsk_parameters_t
+	Gmsk100kFixed6(freq_t frequency_mhz  = 433.45,
+				   tx_params_t tx_params = tx_params_t::Default()) {
+		return {
+			.mod_params =
+				{
+					.bitrate_bps            = 100'000,
+					.frequency_deviation_hz = 25'000,
+					.pulse_shape            = GfskPulseShape::Gauss05,
+					.rx_bandwidth           = GfskRxBandwidth::Bw187200,
+				},
+			.packet_params =
+				{
+					.preamble_bits      = 32,
+					.detector_length    = GfskPreambleDetector::Bits16,
+					.sync_length_bits   = 32,
+					.address_filtering  = GfskAddressFiltering::Disabled,
+					.packet_length_mode = GfskPacketLengthMode::Fixed,
+					.payload_length     = 6,
+					.crc_type           = GfskCrcType::Off,
+					.whitening          = GfskWhitening::On,
+				},
+			.tx_params         = tx_params,
+			.frequency_mhz     = frequency_mhz,
+			.sync_word         = {0x2D, 0xD4, 0xA1, 0x7E, 0, 0, 0, 0},
+			.sync_word_length  = 4,
+			.crc_seed          = 0x1D0F,
+			.crc_polynomial    = 0x1021,
+			.whitening_seed    = 0x0100,
+			.node_address      = std::nullopt,
+			.broadcast_address = std::nullopt,
+		};
+	}
 	void log(const char *tag) const;
 };
 
@@ -656,6 +690,26 @@ struct __attribute__((packed)) fsk_packet_status_t {
 	// the pkt_done IRQ. [negated, dBm, fixdt(0,8,1)] Actual signal power is
 	// –RssiAvg/2 (dBm)
 	uint8_t rssi_avg;
+
+	[[nodiscard]]
+	bool has_error() const {
+		return (rx_status & 0b11111100U) != 0;
+	}
+
+	[[nodiscard]]
+	bool packet_received() const {
+		return (rx_status & 0b00000010U) != 0;
+	}
+
+	[[nodiscard]]
+	auto rssi_sync_dbm() const -> float {
+		return -static_cast<float>(rssi_sync) / 2.0F;
+	}
+
+	[[nodiscard]]
+	auto rssi_avg_dbm() const -> float {
+		return -static_cast<float>(rssi_avg) / 2.0F;
+	}
 };
 
 union packet_status_t {

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(