Exercises the slot-scan logic in event_log_init(): after a simulated
reboot (RAM state cleared, NVS slots preserved) the module must
resume with the correct head/cnt so newest-first read order is
unchanged and subsequent writes continue the seq monotonically.
Adds native-only event_log_test_simulate_reboot() helper. Lifts the
slot-scan loop out of the #ifdef ARDUINO guard so the native stub
exercises the same recovery path as production; the platform-specific
NVS setup remains guarded.
- Remove monotonic counter writes to NVS (stop burning flash on every
event). Derive head and cnt by scanning slots on boot.
- Widen seq to uint32 so slot scan works across multi-year lifetimes.
- Add FreeRTOS mutex around write/read so WiFi event handlers can
safely call event_log_write from another task.
- Check Preferences.begin() return; disable logging if NVS unavailable.
- Extract NTP_SYNC_THRESHOLD constant; drop misleading native uptime.
- Add tests for empty read, max_entries truncation, real-path hash.
Persistent 32-slot ring buffer of tagged diagnostic events (boot, wifi
up/down, http ok/fail, heartbeat miss, reboot). Used to diagnose field
failures post-hoc via the heartbeat payload, without needing serial
access. Native-native stub lets policy be unit-tested.
