Purchase one bounded property search with USDC on Base.

Shared Engine availability for the effective existing filter and type rules. All FIPS arrays are sorted and deduplicated. For resolved supported filters, bindable/unavailable/field-unavailable are disjoint and exhaust requested_fips. Available means all requested counties bindable, partial a nonempty proper subset, unavailable none. Unknown means unresolved filter, with no certified bindable/unavailable/field-unavailable classification; it is never a fallback for missing or corrupt Census authority (generation binding fails). This does not promise visited counties, nonnull fields, distinct parcel totals, or legal access. Optional nullable projections retain existing binding rules.