Move the Nano link to Serial2, update READMEs

- ESP32 talks to the Nano on UART2 (GPIO 17 TX / GPIO 16 RX); Serial (USB)
  is now free for debug output and boot messages no longer reach the Nano
- README: wiring table, actual schedule (clock from 08:00), TODO section
  for the rewiring

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

CLAUDE.md

@@ -23,17 +23,17 @@ See README.md for the overview.
 - **Nano RAM is tight** (ATmega328P, 2 KB). The frame buffers are allocated with
   `calloc`. Avoid `String`, large stack buffers and new globals in Nano code. Put constant
   data in `PROGMEM`.
-- The Nano and the ESP32 share one UART. On the ESP32, `Serial` is the display link, so
-  **don't print debug output on the ESP32's `Serial`**: the Nano would read it as
-  commands. The Nano ignores unknown command bytes, but text that starts with
-  s/S/F/D/P/B/R/T will be run as a command.
+- The ESP32 talks to the Nano on `Serial2` (`nanoSerial` in `libs.h`, GPIO 17 TX / GPIO 16
+  RX). `Serial` (UART0/USB) is free for debug output. Never write debug text to
+  `nanoSerial`: the Nano runs lines that start with s/S/F/D/P/B/R/T as commands.
+- Open hardware TODOs are listed in the README `TODO` section. Update it when one is done.
 - `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 and then sends `READY`. The ESP's `relayOn()` waits for
   that message, at most `NANO_BOOT_TIMEOUT`. The timeout also covers the case where the
   Nano's TX isn't wired to the ESP's RX.
-- `relayOff()` ends `Serial` and sets the TX pin to input. Otherwise the idle-high TX line
+- `relayOff()` ends `nanoSerial` and sets its TX pin to input. Otherwise the idle-high TX line
   powers the Nano through its RX pin. Never send display commands while `relay_on` is
   false.
 - The Nano keeps no state over a power cycle except the flip time in EEPROM. The driver