Best-Execution NBBO Recompute

Recomputes, per supplied fill, price improvement in basis points against the NBBO at execution time, and whether each fill cleared at-or-better than the NBBO. Aggregates the at-or-better rate and average price improvement over a bounded fill set (capped at 5,000 fills). The receipt attests our computation over the fills supplied, not an audit of those fills or a determination of regulatory compliance.