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.

src/meter_driver.cpp

@@ -100,6 +100,52 @@ static bool parse_obis_ascii_unit_scale(const char *line, const char *obis, floa
   return false;
 }
 
+static int8_t hex_nibble(char c) {
+  if (c >= '0' && c <= '9') {
+    return static_cast<int8_t>(c - '0');
+  }
+  if (c >= 'A' && c <= 'F') {
+    return static_cast<int8_t>(10 + (c - 'A'));
+  }
+  if (c >= 'a' && c <= 'f') {
+    return static_cast<int8_t>(10 + (c - 'a'));
+  }
+  return -1;
+}
+
+static bool parse_obis_hex_u32(const char *line, const char *obis, uint32_t &out_value) {
+  const char *p = strstr(line, obis);
+  if (!p) {
+    return false;
+  }
+  const char *lparen = strchr(p, '(');
+  if (!lparen) {
+    return false;
+  }
+  const char *cur = lparen + 1;
+  uint32_t value = 0;
+  size_t n = 0;
+  while (*cur && *cur != ')' && *cur != '*') {
+    int8_t nib = hex_nibble(*cur++);
+    if (nib < 0) {
+      if (n == 0) {
+        continue;
+      }
+      break;
+    }
+    if (n >= 8) {
+      return false;
+    }
+    value = (value << 4) | static_cast<uint32_t>(nib);
+    n++;
+  }
+  if (n == 0) {
+    return false;
+  }
+  out_value = value;
+  return true;
+}
+
 static void meter_debug_log() {
   if (!SERIAL_DEBUG_MODE) {
     return;
@@ -242,6 +288,11 @@ bool meter_parse_frame(const char *frame, size_t len, MeterData &data) {
         p3_ok = true;
         got_any = true;
       }
+      uint32_t meter_seconds = 0;
+      if (parse_obis_hex_u32(line, "0-0:96.8.0*255", meter_seconds)) {
+        data.meter_seconds = meter_seconds;
+        data.meter_seconds_valid = true;
+      }
 
       line_len = 0;
       continue;
@@ -261,6 +312,8 @@ bool meter_parse_frame(const char *frame, size_t len, MeterData &data) {
 }
 
 bool meter_read(MeterData &data) {
+  data.meter_seconds = 0;
+  data.meter_seconds_valid = false;
   data.energy_total_kwh = NAN;
   data.total_power_w = NAN;
   data.phase_power_w[0] = NAN;