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/nano_flash_bridge.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <Arduino.h>
+#include <WiFi.h>
+
+// WiFi to serial bridge for flashing the Nano from the PC (pio run -e nano_ota -t upload).
+//
+// When a TCP client connects, onStart() is called to restart the Nano into its bootloader.
+// After that, bytes are copied between the TCP connection and the Nano's serial port
+// until the client disconnects or is idle too long. onEnd() is then called.
+// The uploader on the PC side is tools/nano_net_upload.py.
+class NanoFlashBridge
+{
+public:
+	NanoFlashBridge(uint16_t port, HardwareSerial &serial, void (*onStart)(), void (*onEnd)());
+
+	void begin();
+
+	// Call from loop(). Returns true while a flash session is running; the caller must not
+	// send anything to the Nano then.
+	bool handle();
+
+private:
+	// Give up when no data moves in either direction for this long
+	static const uint32_t IDLE_TIMEOUT = 10000;
+
+	WiFiServer _server;
+	WiFiClient _client;
+	HardwareSerial &_serial;
+	void (*_onStart)();
+	void (*_onEnd)();
+	bool _active = false;
+	uint32_t _last_activity = 0;
+
+	void _end();
+};