Document that the relay also powers the Nano

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -29,6 +29,10 @@ See README.md for the overview.
   s/S/F/D/P/B/R/T will be run as a command.
 - `src/esp32/secrets.h` holds the real credentials and is gitignored. Never commit it.
   Add new secrets to `secrets.example.h` as placeholders.
+- The relay (ESP32 GPIO 13, `LOW` = on) powers both the panels and the Nano. Each time it
+  switches on, the Nano cold-boots: bootloader, then `begin()` flips every dot on and off.
+  Any data the ESP sends during that time is lost. The Nano keeps no state over a power
+  cycle except the flip time in EEPROM.
 - Hardware is not available here. Builds can be checked, but runtime behaviour can't.
 
 ## Nano serial parser (`src/nano/main.cpp`)