- 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.
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