HID library does not support callbacks

HID should support receiving reports back from the host, at least in a minimal fashion. There's a sketch of what the code might be in the HID library, but it's commented out and not very well fleshed out. It'd be nice if it was at least good enough to handle keyboard LED messages.

https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/libraries/HID/src/HID.cpp

Start with hardware/arduino/avr/libraries/HID/src/HID.cpp and inspect the commented-out callback sketch. Trace how the HID library currently handles reports, then define the minimal host-report path needed for keyboard LED messages. Done means the library can receive and handle those reports without requiring broader HID support.

Written by the indexing model from the issue text.