Registry sources

What a registry source is, how the RSI protocol flows, and the shape of a source's serve loop — the provider side of the registry, built with librsi.

Declare the hives your source backs and register with the kernel to obtain the source fd.

The source serve loop end to end — read a framed request, parse its header, dispatch on the op-code, and decode the payload with its typed parser.

Reply to RSI requests the right way — status-only for most ops and failures, payload-bearing helpers for the five ops that return data, and the validation contract to respect.