feat: USB OTG CDC-ACM with custom VID:PID, disconnect UX

Switch all ESP32-S3 boards from UsbSerialJtag to USB OTG CDC-ACM via
embassy-usb. The USB device stack is now shared code — boards only
provide the platform-specific embassy-usb Driver.

- Custom VID:PID 1209:5741 (open-source VID, "WA" PID)
- CDC-ACM class with proper device enumeration
- COBS+postcard protocol over CDC-ACM packet read/write
- Concurrent USB device loop + protocol loop via join()
- On disconnect: sends DisplayCommand::Reset to show "Waiting for host..."
- Justfile: espflash no longer uses --monitor (OTG steals PHY)
- Test script updated to detect by new VID:PID

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tools/test_ping.py

@@ -12,10 +12,8 @@ import sys
 import time
 from cobs import cobs
 
-# TODO: switch to custom VID:PID 1209:5741 (open-source VID, "WA" PID)
-# once USB OTG CDC-ACM is implemented. For now, ESP32-S3's built-in
-# USB-Serial-JTAG always presents as the Espressif VID:PID.
-USB_VID_PID = "303a:1001"
+# Open-source VID 1209, PID "WA" (0x5741)
+USB_VID_PID = "1209:5741"
 
 
 def find_serial_port() -> str | None: