Add meter Sekundenindex anchoring for epoch timestamps

Parse 0-0:96.8.0*255 meter seconds, derive sample epoch from anchored offset, and detect meter-time jumps via monotonic/delta checks.

include/data_model.h

@@ -26,6 +26,7 @@ struct FaultCounters {
 
 struct MeterData {
   uint32_t ts_utc;
+  uint32_t meter_seconds;
   uint16_t short_id;
   char device_id[16];
   float energy_total_kwh;
@@ -33,6 +34,7 @@ struct MeterData {
   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;