Normalised order book snapshot, returned by every venue package.
bids and asks are sorted by price, best price first:
bidsdescending — highest bid firstasksascending — lowest ask first
Each level is a {price, quantity} tuple. The ordering is part of the contract, not a
convenience: a caller reading hd(bids) as the best bid is reading it correctly, and a
venue package that returns venue-order without re-sorting has broken the contract even
though every value in it is true.
:timestamp is the venue's own, used as-is. :sequence is the venue's book sequence
number where it publishes one, for callers reconciling snapshots against a delta
stream, and nil where it does not.