participant_binding_invalid· reason_code· applies to event_envelope, service_callA `ak.call.state` participant's `participant_binding` failed one of: (a) issuer_kid resolution against current `ak.realm.media_service.service_id`; (b) field consistency with the participant entry (`realm_id` / `call_id` / `focus_id` / `actor_id` / `device_id` / `participant_id`); (c) `expires_at` freshness vs event `created_at`; (d) signature verification. Reducer MUST `failed_precondition`. See zh/crypto-media/call-state.md §4.1.