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/nano/README.md

@@ -8,8 +8,11 @@ logic.
 
 ## Behaviour
 
-- **Boot:** the Nano loads the flip time from EEPROM and flips the whole display on and
-  then off.
+- **Boot:** the Nano loads the flip time from EEPROM, sets up the pins and sends `READY`.
+  It doesn't flip any dots at boot. The relay powers the Nano, so it boots every time the
+  display is switched on.
+- **First `display()` after boot** redraws every dot, because the Nano doesn't know which
+  state the dots were left in.
 - **`display()`:** compares the new buffer with the last one that was shown and strobes
   only the dots that changed.
 - **Every 2 minutes** (`REFRESH_TIME`) it does a full redraw to fix dots that got stuck.
@@ -47,7 +50,10 @@ skipped and the next line starts clean. Coordinates are 2-digit **hex**. Color i
 | `R` | `R` | Print the buffer as ASCII art on the serial port |
 | `T` | `Tnnn` | Set the flip pulse length in µs (decimal, 50–2000), stored in EEPROM |
 
-Invalid commands get a reply that starts with `ERR:` on the Nano's TX pin.
+Replies on the Nano's TX pin:
+
+- `READY` after booting. The ESP waits for it before sending the first frame.
+- `ERR: ...` for invalid commands.
 
 Example (what the ESP32 sends for one pixel and a refresh):