dot/network: implement Light protocol message handling

## Task summary

- currently light protocol messages are not handled (ie they are received and decoded, but that's it)
- need to implement handling of `LightRequest` and `LightResponse` messages

## Specification reference

- this is currently not spec'd so I would say this is not needed for the time being

Start in the dot/network message decoding path and trace where LightRequest and LightResponse are received but not handled. Review the existing message-handling flow and determine the required behavior, since the issue notes that no specification exists yet. Done requires implementing handling for both message types and verifying their behavior.

Written by the indexing model from the issue text.