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Price observations: the scalar market observable pricing consumes.
A Quote is what the pricing layer reads — a mark. It is
deliberately not an order book: trading systems keep the book
(ladders, order-by-order churn) in the execution layer and hand
pricing a derived observable. The book-shaped sibling is
MarketDepth, which lives under
its own market key and produces a Quote at the snapshot boundary.
Derived values (mid, spread, microprice) are functions, never
fields — a stored mid can disagree with the bid/ask it came from;
a computed one cannot. Representation is private by construction
(enum variants are matched, not assigned), so richer quote shapes can
be added without touching pricing call sites: engines only call
mid.