Architecture

Cbox Nexus separates the decision (which it owns) from the data (which it sources behind contracts):

Seam (contract)	Supplies	Default binding
`NexusThresholdSource`	each state's threshold + measurement rules	the `us-tax-data` dataset
`SalesLedger`	the seller's cumulative sales/transactions per state	empty (host binds it)
`PhysicalNexus`	states with asserted physical presence	empty
`NexusRegistrations`	states the seller already registered in	empty

DefaultNexusEngine composes them and produces a NexusEvaluation per state, or a NexusReport across many.

Every host seam carries a NexusSubject — who the question is about — and the engine resolves the threshold before asking the ledger, so the query it passes already carries the state's own measurement terms. Both exist for one reason: a seam should ask a question its implementer can answer without reaching back through the package. See data seams.

Principles:

Contracts-first. Depend on the interfaces; rebind any of them.
Deny-by-default. An unresolvable threshold, a ledger that cannot answer, or totals counted on a different basis than the state measures → Unknown, which needs action — never Below. "I don't know" and "you're fine" are different answers. No physical/registration assertion → none is assumed.
Never infers from one supply. The engine compares host-supplied cumulative totals to thresholds; it does not accumulate sales or watch invoices.