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.cpp

@@ -65,10 +65,6 @@ void BROSE9323::begin(void)
   pinMode(ENABLE, OUTPUT);
   _setData(1);
   _setData(0);
-  fillScreen(1);
-  display();
-  fillScreen(0);
-  display();
 }
 #endif
 
@@ -80,6 +76,11 @@ void BROSE9323::display(bool force)
 #else
   if (_direct_mode)
     return;
+  if (_force_next)
+  {
+    force = true;
+    _force_next = false;
+  }
   for (uint8_t x = 0; x < width(); x++)
   {
     _selectPanel(x / _panel_width);
@@ -188,6 +189,7 @@ void BROSE9323::fillScreen(uint16_t color)
     }
 #if !defined(__AVR_ATmega168P__) && !defined(__AVR_ATmega168PB__) && !defined(__AVR_ATmega168__)
     memset(_old_buffer, color ? 0xFF : 0x00, _buffer_size);
+    _force_next = false;
 #endif
   }
 #endif

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,