descriptor = {
  id: "residency_country",          // machine-readable key
  label: "Country of Residence",    // human label
  type: "enum",                     // scalar | string | enum | range | bool | ref
  format: "ISO-3166-1 alpha-2",     // optional, e.g. PT, FR, US
  issuer_policies: {
    allowed_issuers: ["self", "gov", "institution"],
    requires_attestation: true      // needs external proof? (e.g. gov-issued)
  },
  validity_policy: {
    max_duration_days: 365,         // suggested expiry window
    require_refresh_on_change: true // must refresh proofs if value changes
  },
  proof_templates: [
    "is_over_18",
    "lives_in_EU"
  ]
}