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>

lib/BROSE9323/BROSE9323.h

@@ -24,6 +24,8 @@ private:
     uint8_t _active_col = 255;
     uint8_t _active_row = 255;
     bool _active_data;
+    // After boot the physical dot state is unknown, so the first display() redraws everything
+    bool _force_next = true;
 
 #ifndef FLIPDOT_PLCC_ADAPTER
     const uint8_t ENABLE = 7,