Optional expected-payer pin. When set, settle
refuses any other claimed payer and requires the settling
transaction to contain a USDC Transfer from this address.

Opaque client-chosen key dedupes a state-changing request for 24 hours.
Required on POST /v1/payments; optional on every other mutation. A
replay with the same key and the same request hash returns the cached
response; a replay with the same key but a different hash returns
409 idempotency.conflict.

Selects the network slice the request targets. Workspace-scoped
responses are filtered to this network; chain-bound writes are
stamped with it. Required on every non-admin workspace-scoped
operation.