test: add json stability and discovery payload coverage

lib/dd3_legacy_core/include/data_model.h

@@ -0,0 +1,60 @@
+#pragma once
+
+#include <Arduino.h>
+
+enum class FaultType : uint8_t {
+  None = 0,
+  MeterRead = 1,
+  Decode = 2,
+  LoraTx = 3
+};
+
+enum class RxRejectReason : uint8_t {
+  None = 0,
+  CrcFail = 1,
+  InvalidMsgKind = 2,
+  LengthMismatch = 3,
+  DeviceIdMismatch = 4,
+  BatchIdMismatch = 5,
+  UnknownSender = 6
+};
+
+struct FaultCounters {
+  uint32_t meter_read_fail;
+  uint32_t decode_fail;
+  uint32_t lora_tx_fail;
+};
+
+struct MeterData {
+  uint32_t ts_utc;
+  uint32_t meter_seconds;
+  uint16_t short_id;
+  char device_id[16];
+  float energy_total_kwh;
+  float phase_power_w[3];
+  float total_power_w;
+  float battery_voltage_v;
+  uint8_t battery_percent;
+  bool meter_seconds_valid;
+  bool valid;
+  int16_t link_rssi_dbm;
+  float link_snr_db;
+  bool link_valid;
+  uint32_t err_meter_read;
+  uint32_t err_decode;
+  uint32_t err_lora_tx;
+  FaultType last_error;
+  uint8_t rx_reject_reason;
+};
+
+struct SenderStatus {
+  MeterData last_data;
+  uint32_t last_update_ts_utc;
+  uint32_t rx_batches_total;
+  uint32_t rx_batches_duplicate;
+  uint32_t rx_last_duplicate_ts_utc;
+  bool has_data;
+};
+
+void init_device_ids(uint16_t &short_id, char *device_id, size_t device_id_len);
+const char *rx_reject_reason_text(RxRejectReason reason);

lib/dd3_legacy_core/include/json_codec.h

@@ -0,0 +1,6 @@
+#pragma once
+
+#include <Arduino.h>
+#include "data_model.h"
+
+bool meterDataToJson(const MeterData &data, String &out_json);