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>

Cargo.lock

@@ -347,6 +347,7 @@ dependencies = [
  "embassy-sync 0.6.2",
  "embassy-time",
  "embassy-usb",
+ "embassy-usb-driver",
  "embedded-graphics",
  "embedded-io-async",
  "esp-backtrace",