An outbound webhook destination URL, validated at EVERY write path by living in the type: http(s) scheme, host not a known metadata name, and a literal-IP host (v4, v6, mapped/compatible v6) never in a private/loopback/link-local/reserved range (ADR-0005's registration-time floor — deterministic and offline-safe).

DNS resolution of HOSTNAME urls is deliberately NOT done here: the ADR assigns send-time DNS re-resolution to the delivery runtime, and cast-time resolution would make every changeset network-dependent.