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>

platformio.ini

@@ -1,7 +1,8 @@
 ; Flipdot display - one PlatformIO project, two firmware targets.
 ;
-;   nano       Arduino Nano that drives the Brose flipdot panels directly
-;   esp32      ESP32 "brain" (WiFi, NTP clock, Home Assistant) - OTA upload
+;   nano       Arduino Nano that drives the Brose flipdot panels directly (USB upload)
+;   nano_ota   Same Nano firmware, flashed over WiFi through the ESP32
+;   esp32     ESP32 "brain" (WiFi, NTP clock, Home Assistant) - OTA upload
 ;   esp32_usb  Same ESP32 firmware, flashed over USB (first flash / recovery)
 ;   native     Unit tests for the Nano firmware, run on the PC (needs gcc on PATH)
 ;
@@ -38,6 +39,14 @@ build_src_filter = +<nano/>
 upload_port = COM10
 monitor_port = COM10
 
+; Same Nano firmware, flashed over WiFi through the ESP32 (see README.md).
+; Needs the Nano TX -> ESP32 GPIO 16 wire. upload_port = ESP32 IP[:port], default port 2323.
+[env:nano_ota]
+extends = env:nano
+upload_protocol = custom
+upload_port = 10.1.1.198
+upload_command = "$PYTHONEXE" "$PROJECT_DIR/tools/nano_net_upload.py" "$SOURCE" $UPLOAD_PORT
+
 ; ---------------------------------------------------------------------------
 ; ESP32 - controller, talks to the Nano over serial
 ; ---------------------------------------------------------------------------