resolver: a domain's nameservers are only its own delegation (closes #222)

LookupNS now follows the delegation for the domain alone. When the parent
zone's servers answer that it has no delegation, as for a domain that does
not exist, the set is empty, and the watcher's NS comparison reports every
nameserver removed.

FindAuthoritativeNameservers, used for hostnames, still moves to a parent
name when the servers answer that the name has no delegation of its own,
but returns the error when they do not answer, where it used to take a
parent zone's nameservers. The fallback walk treats an authoritative
answer the same way.

A domain with no delegation still has its own records asked at the
servers of the zone it is in.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-02: a domain that does not exist has no nameservers, not its parent
+  zone's; no name gets a parent's when its servers did not answer (closes #222).
 - 2026-10-02: a name removed from `DNSWATCHER_TARGETS` leaves the state, and so
   the dashboard and API, at startup, before the first check (closes #223).
 - 2026-10-02: a record type whose query to a nameserver fails keeps its previous