fix: sort MST records by full key for correct byte ordering
Records must be sorted by their full MST key (collection/rkey) in byte
order, not by collection then rkey separately. This matters when
collection names have prefix relationships - e.g. "social.grain.gallery"
vs "social.grain.gallery.item" - because '.' (46) < '/' (47) in ASCII.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>