Support both Nano bootloaders for flashing over WiFi

- Uploader sends 'FLASH <baud>' first; the ESP32 restarts the Nano and
  bridges at that baud rate, then answers OK
- --baud=auto (default) tries 57600 (old ATmegaBOOT) then 115200 (Optiboot)
- Optional nano_reset GPIO (NPN to Nano RESET) pulses reset after power
  on, needed for Optiboot which skips the bootloader after a power cycle
- Clear errors for outdated ESP32 firmware and missing wires
- README: bootloader table, reset wire, troubleshooting

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

src/esp32/nano_flash_bridge.h

@@ -5,14 +5,16 @@
 
 // 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
+// The uploader (tools/nano_net_upload.py) connects and first sends one line:
+//   FLASH <baud>\n
+// with the baud rate of the Nano's bootloader (57600 old bootloader, 115200 Optiboot).
+// onStart(baud) then restarts the Nano into its bootloader, the bridge answers "OK\n",
+// and from then on 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)());
+	NanoFlashBridge(uint16_t port, HardwareSerial &serial, void (*onStart)(uint32_t baud), void (*onEnd)());
 
 	void begin();
 
@@ -21,16 +23,19 @@ public:
 	bool handle();
 
 private:
+	// Max wait for the FLASH line after connecting
+	static const uint32_t HEADER_TIMEOUT = 2000;
 	// 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 (*_onStart)(uint32_t baud);
 	void (*_onEnd)();
 	bool _active = false;
 	uint32_t _last_activity = 0;
 
+	uint32_t _readHeader(WiFiClient &client);
 	void _end();
 };