watcher: a removed nameserver is an NS Change, not an NS Failure (closes #262)

A nameserver that left a domain's delegation was reported twice: as the
domain's NS Change, and by the check of the domain's own records as an
error-level NS Failure saying it disappeared. The check of a name's
records now sends one NS Change at warning level, starting Hostname: or
Domain:, listing the nameservers it was asked at before and not now.
For a domain it leaves out those of its saved delegation, which the
domain's own NS Change lists, so a domain's new delegation is now
notified and saved only after, and only together with, its records.
Nothing is sent for a nameserver that joins.

A nameserver that was failing before it left is now listed too; the
NS Failure skipped it.

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
 
+- 2026-10-13: a nameserver leaving a domain's delegation sends only NS Change;
+  one leaving a hostname's zone sends NS Change, not NS Failure (closes #262).
 - 2026-10-13: a save syncs the state file to disk before renaming it into place,
   so a crash cannot leave it empty and stop the next start (closes #264).
 - 2026-10-13: a `PORT` that cannot be bound stops startup with an error naming