Hacker-friendly USB host to SPI converter

This project is currently on hold as I try to narrow my focus.

There is an interesting alternative in https://github.com/No0ne/ps2pico.

https://docs.arduino.cc/retired/shields/arduino-usb-host-shield MAX3421E

https://hackaday.io/project/13273-diy-vt100-a-miniature-hardware-terminal: USB host keyboard with STM32F767VxTx

https://www.hobbytronics.co.uk/usb-host-keyboard (PIC24F-based, non-free)

https://www.circuitsworld.com/usb-keyboard-decoder

https://www.versalent.biz/wlkeyboardv2.htm WLKB232V2