feat: implement USB serial with COBS-framed postcard protocol

Wire up UsbSerialJtag on ESP32-S3 boards for host communication:
- Read COBS-framed postcard commands from host via serial
- Route DisplayOn/Off to display task, everything else to radio
- Serialize responses back to host as COBS frames
- Use postcard::CobsAccumulator for chunked deserialization
- Concurrent read/write via embassy select

RAK nRF52840 USB (embassy-usb CDC-ACM) is still a stub — different
init path, will be implemented separately.

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

Cargo.lock

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