docs: add DS3231 RTC to Hardware section (BUG-18)

PLAN.md

@@ -29,6 +29,13 @@ pick up the project without needing prior chat history.
 - **Power**: Hard-wired 5V/GND directly to GPIO header (red to Pin 2, black to Pin 6), bypassing
   the worn micro-USB connector. `vcgencmd get_throttled` returns `0x0` — no undervoltage or
   throttling. Pi runs at full 1.2 GHz (was ~600 MHz downclocked before the fix).
+- **RTC**: **DS3231** module (BUG-18, installed 2026-08-14) on the free I²C bus `i2c-1` — SDA
+  pin 3, SCL pin 5, VCC pin 1 (3.3V, *not* 5V), GND pin 6. ZS-042 board's charge diode removed
+  for the non-rechargeable CR2032 backup cell. Enabled via `dtparam=i2c_arm=on` +
+  `dtoverlay=i2c-rtc,ds3231` in `/boot/firmware/config.txt`, `fake-hwclock` disabled, `i2c-dev`
+  in `/etc/modules-load.d/i2c-dev.conf`. Probes at `0x68` (driver → `UU` in i2cdetect), 24C32
+  EEPROM at `0x57`; power-cycle test boots with the correct time (was `2000-01-01`). SIM868
+  HAT's CR1220 also installed and its RTC set via `AT+CCLK` as a secondary backup.
 - **Car**: 2013 Toyota Yaris.
 
 ### SIM868 HAT bring-up notes (important gotchas)