- Add SSD1306 128x64 OLED display with attenuation bar, dB readout,
step counter, sweep indicator, and WiFi RSSI
- Switch all debug output to Serial0 (UART0 via CH343) for consistent
serial comms when USB CDC is not used
- Remove unused USB.h includes from all source files
- Add development notes to CLAUDE.md (no stty, serial config docs)
