docs: BUG-18 DS3231 installed + working (I²C 0x68), pending power-cycle test

PLAN.md

@@ -2273,11 +2273,12 @@ logged as `BUG-XX` entries in [BUGS.md](./BUGS.md) (BUG-16 onward) and fixed + v
   across reboots; also fixed a NULL-GPS speed-only row crash in the trip builder. Verified the
   2026-08-13 drive now appears locally.
 - **BUG-17 (trip timestamps wrong) — OPEN**: no RTC → clock drifts without internet.
-- **BUG-18 (no RTC) — OPEN**: plan set — DS3231 RTC on I²C (desolder charging circuit, CR2032)
-  + SIM868 CR1220 RTC backup; NTP online, RTC/GNSS fallback offline.
+- **BUG-18 (no RTC) — 🚧 IN PROGRESS**: DS3231 installed + configured + detected on I²C (`0x68`),
+  `hwclock` reads correct time; `fake-hwclock` disabled; `i2c-dev` persisted. Pending the
+  power-cycle persistence test. Secondary: SIM868 CR1220 RTC backup still to install.
 
 Open items from the drive — fill in as reported:
-- BUG-17 / BUG-18 (clock) pending.
+- BUG-17 (clock) — blocked on BUG-18 power-cycle verification.
 
 ## Working Notes / Environment Gotchas