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.toml

@@ -13,6 +13,7 @@ embassy-executor = { version = "0.6", features = ["task-arena-size-32768"] }
 embassy-sync = "0.6"
 embassy-time = "0.3"
 embassy-usb = { version = "0.3", features = ["defmt"] }
+embassy-usb-driver = "0.1"
 heapless = { version = "0.8", features = ["serde"] }
 lora-phy = "3.0"
 postcard = { version = "1.0", default-features = false }