Tool contract 7.2.3

Board collections say how old they are (feedback #322-#323).

• collections_get on a board collection (synced_from) serves synced_at (the last sync), synced_snapshot_observed_at (the board snapshot that sync applied) and board_observed_at (the board's newest). When the board has moved since the sync, a note says the newer board is not applied yet and points at rankings_players; the note no longer claims "today's membership" outright (0172).
• A collection segment on the meta tools dates the membership it applies (the board as synced).

Additive.