watcher: notify NS query failure and recovery (closes #104)

LookupAllRecords now returns each nameserver's response, so the
watcher saves its status: ok when it answered, NXDOMAIN and no records
included, and error with the reason when it timed out, answered
SERVFAIL or REFUSED, or could not be reached. A nameserver that starts
failing sends NS Failure and one that answers again sends NS Recovery.
A failing nameserver is left out of the record change and
inconsistency comparisons. The resolver used to report REFUSED and
network errors as an answer with no records; they are now errors. A
lookup cut short by its context now returns an error instead of a
failure of the nameserver it was querying.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
+- 2026-10-01: a nameserver that does not answer is saved as `error` with the
+  reason, and NS failure and NS recovery are notified (closes #104).
 - 2026-09-29: the live-DNS test package is renamed `internal/livednstest` and
   added to the `test-support` `deny` list in `.golangci.yml`, so `make lint`
   fails when program code imports it (closes #164).