collections.userVisibleLibraryIDs keeps its identity adapter and
delegates; the router's visibleLibraryIDsFor likewise (its identity
source is the session user_id string, the site-specific part it keeps).
The SSR site made the unification slog-proof: resolveLibrary — the SSR
one-of-a-kind (active selection + switcher rows + offline detection) —
stays untouched per the semantic diff.