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

BUGS.md

@@ -19,10 +19,19 @@ When CLOSED, move the entire entry to Resolved — don't duplicate.
   timestamp jump mid-trip. Root cause is BUG-18.
 
 ### BUG-18 — Pi needs offline timekeeping (no RTC)
-- **Status**: `OPEN` 2026-08-13
+- **Status**: `IN PROGRESS` 2026-08-14
 - **Phase**: 29
 - **Problem**: the Pi 3 has no real-time clock. Without internet (no NTP) the system clock
   is wrong across power cycles, corrupting tracking/trip timestamps (BUG-16/BUG-17).
+- **Progress 2026-08-14**: DS3231 module installed + working. Pi config done:
+  `dtparam=i2c_arm=on` + `dtoverlay=i2c-rtc,ds3231` in `/boot/firmware/config.txt`,
+  `i2c-tools` installed, `fake-hwclock` disabled, `i2c-dev` added to `modules-load.d`.
+  The DS3231 now probes at `0x68` (`UU`), its 24C32 EEPROM at `0x57`, `/dev/rtc0` registered,
+  and `hwclock -r` matches system time. (First scan was an empty bus — a data-line wiring
+  issue, since resolved.)
+  **Pending**: power-cycle persistence test — unplug, replug, confirm correct time at boot
+  without NTP. Also: the charge diode was re-added during debugging and must be removed again
+  for the CR2032.
 - **Fix plan (both, they complement each other)**:
   1. **DS3231 RTC module (primary)** — battery-backed RTC on the free I²C bus (SDA pin 3,
      SCL pin 5, 3.3V pin 1, GND pin 6). Enable `dtparam=i2c_arm=on` +

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