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/README.md

@@ -14,6 +14,7 @@ debug output. Wiring is described in the [main README](../../README.md#wiring-be
 | `main.cpp` | setup, OTA, clock / power logic, Home Assistant callbacks |
 | `libs.h` | includes and global objects (display, MQTT, WiFi, pins, timezone) |
 | `bitmaps.h` | 1-bit bitmaps (Pikachu, thought bubble, Nintendo logo, ...) |
+| `nano_flash_bridge.h/.cpp` | WiFi to serial bridge for flashing the Nano over WiFi (TCP port 2323) |
 | `secrets.example.h` | template for `secrets.h` (WiFi and MQTT credentials, **gitignored**) |
 
 ## Features
@@ -34,6 +35,9 @@ debug output. Wiring is described in the [main README](../../README.md#wiring-be
   - **Off:** the ESP stops sending and releases its TX pin (`Serial.end()` and the pin set
     to input). An idle UART line is high and would otherwise power the Nano through its RX
     pin.
+- **Nano flash bridge:** `pio run -e nano_ota -t upload` flashes the Nano through the ESP32.
+  While a flash session runs, the clock is paused. See the
+  [main README](../../README.md#flashing-the-nano-over-wifi).
 - **Status LED** (built-in) blinks fast when WiFi is down and slowly when connected. It is
   off at night or when the display is powered off.