Function dns_record_payload 

pub fn dns_record_payload(rec: &FrontDoorRecord) -> Value

Shape the Cloudflare POST /zones/{zone}/dns_records body for a front-door record.

A TXT record at _qdp.<domain> whose content is the compact front-door value. TTL 300s (5 min) — short, because the front door is meant to be updatable quickly.