Line-based JSON protocol over the ESP32-S3 native USB OTG port,
providing deterministic sub-millisecond attenuator control without
WiFi interference. Runs alongside the existing REST API.
Commands: identify, status, config, set, sweep, sweep_stop
Protocol: usb-serial-json-v1 (one JSON object per \n-terminated line)
Also addresses pre-existing reliability issues found during review:
- Thread safety: FreeRTOS mutex in Attenuator class (web server
callbacks run on async_tcp task, not loop())
- NVS flash wear: skip persist during sweep, save on stop
- WiFi credentials: moved to gitignored platformio_local.ini
- Shared header: sweep.h replaces duplicated extern declarations
