Function parse_book_snapshot 

pub fn parse_book_snapshot(
    snap: &PolymarketBookSnapshot,
    instrument_id: InstrumentId,
    price_precision: u8,
    size_precision: u8,
    ts_init: UnixNanos,
) -> Result<OrderBookDeltas>

Parses a book snapshot into [OrderBookDeltas] (CLEAR + ADD).

A book with no resting orders is a valid snapshot: it parses to a lone CLEAR so the baseline is accepted instead of burning recovery budget.