Bookable spot data for a session: the layout (rows/columns/style and the spots themselves) plus viewer-relative spot ID sets reflecting the viewer's relationship to each occupied spot. takenIds is the union of all confirmed registrations on the session. viewerIds, viewerGuestIds, and viewerBookerIds are subsets of takenIds: spots the viewer holds for themselves, spots the viewer holds for a guest, and spots held by users who booked the viewer as a guest, respectively. All viewer sets are empty for unauthenticated requests.