build_receiver_feedback normalises the record a receiver sends back —
damage lists sorted and unique, chunk keys well-formed, a keyframe
requested whenever something is missing — and until now lived in Muninn,
the one end that never sends it. Ratatoskr sends it; Muninn's sender
tests construct it to exercise the repair cache. A builder both ends
call belongs beside the record it builds.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>