Call an explicitly selected capability

Send an explicit capability id. Unpaid x402 reservations expire after one hour; retry with a new idempotency key. Available referral credit is applied automatically before another payment rail. Signed-in people reserve their existing card top-up balance at $1 per credit; success captures the hold and failure releases it. Verified people receive a one-time $3 capability-only trial, spent before paid balance and restored only as trial credit after failure. GET /v1/capabilities/{id}/call returns the signed-in person's available paid and trial balance. Send Idempotency-Key; body idempotency_key is accepted when that header is absent. A root endpoint receives a call id and a short-lived composition token for that dispatch. Registered agents use x402, signed-in people and API-key buyers use balance, or the root supplier uses upstream with the parent id and composition token. Upstream is limited to one level: the child list price is deducted from the parent supplier accrual and is not added to the buyer price. Verified people arriving with a supplier via token receive the same first-contact attribution checks and 3% brought-demand fee; the web form preserves the token through sign-in.