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>

src/esp32/README.md

@@ -26,7 +26,13 @@ BROSE9323 driver is built with `-D ESPSERIAL`, so every GFX call is sent over `S
   - `FlipDotDisplayClock`: clock on, or the Nintendo logo when off
   - `FlipDotDisplayPower`: turns the display off through the relay
 - **Relay on GPIO 13** switches the power for both the panels and the Nano. `LOW` turns it
-  on. After switching on, the ESP waits 3 s so the Nano can boot before frames are sent.
+  on.
+  - **On:** the ESP waits for the Nano's `READY` message, at most 3 s
+    (`NANO_BOOT_TIMEOUT`), before it sends frames. The relay stays on while it is needed;
+    the ESP only waits when it actually switches the relay on.
+  - **Off:** the ESP stops sending and releases its TX pin (`Serial.end()` and the pin set
+    to input). An idle UART line is high and would otherwise power the Nano through its RX
+    pin.
 - **Status LED** (built-in) blinks fast when WiFi is down and slowly when connected. It is
   off at night or when the display is powered off.