DCQL: claim sets (preferred claim with fallback)

Scenario

A verifier needs an age check. It prefers the minimal boolean is_over_21; if the wallet's PID lacks it, the query falls back to birthdate. claim_sets are ordered - the wallet satisfies the first set it can.

Prerequisites

• An iGrant.io Organisation Wallet Suite (OWS) API key. Get it from support@igrant.io. Keep it on the server, in an environment variable or a secret manager. The browser never sees it.
• The OWS environment the key belongs to. The default is demo (https://demo-api.igrant.io). Use staging (https://staging-api.igrant.io) only when the integrator asks for it. A key works only in its own environment.