watcher: keep port state when no nameserver of a name answered (closes #193)

The port check removed the saved port state of every address no
configured name resolves to. A name whose nameservers all timed out
or failed is saved with no records, so its addresses looked gone and
lost their port state; when the nameservers answered again it was
recorded afresh, and a port that opened or closed meanwhile was not
notified.

An entry is now kept when one of the names saved on it is configured
and none of its nameservers answered on its last check, and such a
name stays on the entry when the port is checked again for another
name. A name whose nameservers answer with no addresses still loses
it, and so does a name no longer configured.

Model: opus-5-5

TODO.md

@@ -20,6 +20,8 @@ https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-01: when none of a configured name's nameservers answered, the port
+  state saved for its addresses is kept, not removed (closes #193).
 - 2026-10-01: `make fmt-check` fails on a file `goimports` would change; both
   format scripts run `goimports` at its pinned commit, not from `PATH` (#119).
 - 2026-10-01: a hostname is queried at the servers of the zone it is in, found