Power-cycle-friendly Nano boot and ESP relay handling

Nano / driver:
- No full on/off flip at boot; the first display() after boot is a forced
  full redraw instead, since the physical dot state is unknown
- Send READY when booted

ESP32:
- relayOn() only switches and waits when the relay was off, and waits for
  the Nano's READY (3 s timeout) instead of a fixed 3 s on every frame
- relayOff() ends Serial and releases the TX pin so the idle-high line no
  longer back-powers the Nano through its RX pin
- No display commands are sent while the relay is off
- Switching the relay on cancels a pending power-off countdown

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

CLAUDE.md

@@ -30,9 +30,14 @@ See README.md for the overview.
 - `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.
+  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
+  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
+  forces a full redraw on the first `display()` after boot (`_force_next`).
 - Hardware is not available here. Builds can be checked, but runtime behaviour can't.
 
 ## Nano serial parser (`src/nano/main.cpp`)
@@ -45,8 +50,9 @@ See README.md for the overview.
   2 s with no serial data. The ESP sends one frame per second, ending with `D`.
 - Flip time is stored in EEPROM as a magic word at address 0 and a `uint16_t` at
   address 2. Values outside 50–2000 µs are rejected.
-- The Nano doesn't send an acknowledgement. Whether the Nano's TX is wired to the ESP's RX
-  is unknown, so don't make the ESP wait for replies.
+- The Nano only replies `READY` (at boot) and `ERR: ...`. There is no per-frame
+  acknowledgement. Whether the Nano's TX is wired to the ESP's RX is unconfirmed, so every
+  wait for a reply needs a timeout.
 - To test the parser without hardware, compile `main.cpp` with g++ (Strawberry Perl
   provides it at `C:\Strawberry\c\bin`) and mock headers for `Arduino.h`, `EEPROM.h` and
   `BROSE9323.h`.