Flash the Nano over WiFi through the ESP32

- ESP32: NanoFlashBridge on TCP port 2323. A connection power cycles the
  Nano into its bootloader and bridges TCP <-> Serial2 at 57600 baud; the
  clock pauses during the session, 10 s idle timeout
- tools/nano_net_upload.py: STK500v1 uploader over TCP (standard library
  only), with sync retries, write and read-back verification. avrdude 6.3
  net: ports are broken on Windows, so avrdude is not used
- New nano_ota environment: pio run -e nano_ota -t upload
- README: usage, requirements, troubleshooting; TODO for hardware test

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

src/esp32/main.cpp

@@ -39,7 +39,7 @@ void relayOn()
   if (relay_on)
     return;
 
-  nanoSerial.begin(115200, SERIAL_8N1, nano_rx, nano_tx);
+  nanoSerial.begin(NANO_BAUD, SERIAL_8N1, nano_rx, nano_tx);
   // drop old data (e.g. ERR replies) so only a new READY counts
   while (nanoSerial.available())
     nanoSerial.read();
@@ -59,6 +59,30 @@ void relayOff()
   nanoSerial.end();
   pinMode(nano_tx, INPUT);
 }
+
+// Flash session started: power cycle the Nano so its bootloader runs, and talk to the
+// bootloader at its baud rate. The PC uploader keeps sending sync bytes until it answers.
+void startNanoFlash()
+{
+  if (relay_on)
+    relayOff();
+  delay(NANO_POWER_OFF_TIME);
+
+  nanoSerial.begin(NANO_BOOTLOADER_BAUD, SERIAL_8N1, nano_rx, nano_tx);
+  while (nanoSerial.available())
+    nanoSerial.read();
+  digitalWrite(relay, LOW);
+}
+
+// Flash session ended: switch off, the clock powers the Nano on again with the new firmware
+void endNanoFlash()
+{
+  relayOff();
+  // skip the frames that were missed while flashing
+  next_display = millis();
+}
+
+NanoFlashBridge nanoFlashBridge(NANO_FLASH_PORT, nanoSerial, startNanoFlash, endNanoFlash);
 void turnPowerOff(uint32_t delay = 30000, bool _turn_hassio_power_off = false)
 {
     if(triggered_power_off == false)
@@ -269,6 +293,7 @@ void setup()
                });
 
   ArduinoOTA.begin();
+  nanoFlashBridge.begin();
 
   do
   {
@@ -441,6 +466,11 @@ void loop()
 {
   ArduinoOTA.handle();
   mqtt.loop();
+
+  // While the Nano is being flashed, nothing else may talk to it
+  if (nanoFlashBridge.handle())
+    return;
+
   updatePower();
 
   wifiStatus = WiFi.waitForConnectResult() == WL_CONNECTED;