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.toml

@@ -23,7 +23,8 @@ static_cell = "2.1"
 embedded-graphics = "0.8"
 ssd1306 = { version = "0.10", features = ["async"] }
 
-# Async combinators (select)
+# Async IO traits and combinators
+embedded-io-async = "0.6"
 embassy-futures = "0.1"
 
 # ESP32-S3 (Heltec V3) — requires esp toolchain (espup)