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

When a domain's parent zone's servers answer NXDOMAIN, LookupNS returns
ErrNXDomain. The watcher then saves the domain with no nameservers and
nxdomain set, shown on the dashboard and in /api/v1/status, asks for none
of its records and removes those saved, so its old nameservers go in one
NS Change. A domain with no delegation of its own gets an empty set and
its records are still asked at the zone it is in.
FindAuthoritativeNameservers moves to a parent name only on one of those
two answers; when the servers do not answer, it returns the error. After
an upgrade, a domain without its own delegation that was saved with its
parent zone's nameservers gets one NS Change; the README says so.

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 is shown so, with no nameservers; no
+  name gets a parent's nameservers when its own did not answer (closes #222).
 - 2026-10-02: the refused-query test sends one query to four operators' public
   resolvers in turn until one replies, not eight to one operator (closes #251).
 - 2026-10-02: a name removed from `DNSWATCHER_TARGETS` leaves the state, and so