resolver: ask a referral's nameservers that come without addresses (closes #221)

Looking up a nameserver's own address followed only the addresses a
referral gave, so a nameserver whose zone is delegated without them,
such as a.ntpns.org of pool.ntp.org, never resolved. The walk to a
name's nameservers looked addresses up only when a referral gave none.
Both now go through queryZone, which asks the nameservers whose
addresses the referral gives first and, if none of them gives a usable
reply, looks up and asks the others. maxLookupDepth stops lookups three
deep, so delegations that point at each other still end; when the limit
is why no address was found, the error is ErrLookupDepthExceeded, not
"no address".

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: nameservers a referral names without addresses are looked up,
+  three deep at most; `pool.ntp.org`'s nameservers resolve (closes #221).
 - 2026-10-02: an apex domain is not counted or listed as a hostname; its records
   show under Domains, and notifications about them say `Domain:` (closes #224).
 - 2026-10-02: the dashboard lists each nameserver's record types in one fixed