add hardware RTC startup sync and periodic system clock correction

AutoDiscoverRTCClock now syncs the fallback system clock from the
hardware RTC immediately after discovery in begin(), and re-syncs
every hour via tick() to correct drift. Invalid RTC readings (before
2024-01-01) are ignored to handle unpowered DS3231 gracefully.

src/helpers/AutoDiscoverRTCClock.cpp

@@ -26,6 +26,17 @@ bool AutoDiscoverRTCClock::i2c_probe(TwoWire& wire, uint8_t addr) {
   return (error == 0);
 }
 
+void AutoDiscoverRTCClock::syncSystemClock() {
+  uint32_t hw_time = getCurrentTime();
+  if (hw_time <= 1704067200UL) return;  // DS3231 not set or lost power (before 2024-01-01)
+
+  uint32_t sys_time = _fallback->getCurrentTime();
+  int32_t drift = (int32_t)(hw_time - sys_time);
+  if (drift > 2 || drift < -2) {
+    _fallback->setCurrentTime(hw_time);
+  }
+}
+
 void AutoDiscoverRTCClock::begin(TwoWire& wire) {
   if (i2c_probe(wire, DS3231_ADDRESS)) {
     ds3231_success = rtc_3231.begin(&wire);
@@ -49,6 +60,12 @@ void AutoDiscoverRTCClock::begin(TwoWire& wire) {
     rtc_8130_success = true;
     MESH_DEBUG_PRINTLN("RX8130CE: Initialized");
   }
+
+  _has_hw_rtc = ds3231_success || rv3028_success || rtc_8563_success || rtc_8130_success;
+  if (_has_hw_rtc) {
+    syncSystemClock();
+    _last_sync_ms = millis();
+  }
 }
 
 uint32_t AutoDiscoverRTCClock::getCurrentTime() {

src/helpers/AutoDiscoverRTCClock.h

@@ -4,12 +4,20 @@
 #include <Arduino.h>
 #include <Wire.h>
 
+#ifndef RTC_RESYNC_INTERVAL_MS
+  #define RTC_RESYNC_INTERVAL_MS  3600000UL   // re-sync system clock from hardware RTC once per hour
+#endif
+
 class AutoDiscoverRTCClock : public mesh::RTCClock {
   mesh::RTCClock* _fallback;
+  bool _has_hw_rtc;
+  unsigned long _last_sync_ms;
 
   bool i2c_probe(TwoWire& wire, uint8_t addr);
+  void syncSystemClock();
 public:
-  AutoDiscoverRTCClock(mesh::RTCClock& fallback) : _fallback(&fallback) { }
+  AutoDiscoverRTCClock(mesh::RTCClock& fallback)
+    : _fallback(&fallback), _has_hw_rtc(false), _last_sync_ms(0) { }
 
   void begin(TwoWire& wire);
   uint32_t getCurrentTime() override;
@@ -17,5 +25,12 @@ public:
 
   void tick() override {
     _fallback->tick();   // is typically VolatileRTCClock, which now needs tick()
+    if (_has_hw_rtc) {
+      unsigned long now = millis();
+      if ((unsigned long)(now - _last_sync_ms) >= RTC_RESYNC_INTERVAL_MS) {
+        syncSystemClock();
+        _last_sync_ms = now;
+      }
+    }
   }
 };