Empty today. It names the append site for the first field a responder needs —
a coverage hint, a record age, a freshness bound — so that field lands as a
pure append rather than a cdn/dht/v2 bump. Postcard fills no defaults for
absent trailing fields, which is why the field must go in a separately encoded
extension rather than onto FindValueResponse itself.
Anything added here is UNSIGNED and responder-controlled. DHT records carry
no per-record signature (see the module docs), so an extension field is a
hint the requester may act on only where a lying responder gains nothing.