A CNAME (or MX/SRV/NS) whose target is "@" was being concatenated
literally as "@.<apex>" instead of resolving to the apex itself.
Replace the hand-rolled normaliseTarget logic with sdk.JoinRelative,
which correctly maps "" and "@" to the origin. Absolute FQDNs
(trailing dot, used for external targets) are still stripped before
the call so the external-target path is unaffected.
happyDomain encodes service-embedded record owners relative to the
zone apex, so returning the trimmed Hdr.Name as the FQDN owner left
relative names like "_465._tcp" leaking into pointer dedup keys and
discovery entries. Join with apex unless the name is already
suffix-anchored.
