watcher: send the inconsistency alert once per disagreement (closes #158)

detectInconsistencies alerted for neighbouring pairs of nameservers whose
records differed, on every DNS check, for as long as they differed. It now
also takes the previous hostname state, compares every pair of nameservers,
and alerts for a pair that differs unless both were in that state and
already differed there, so a nameserver new on a check that answers
differently is reported once. The state loaded at startup is the previous
state for the first check, so a disagreement saved before a restart is not
reported again. The choice of pairs is tested on record data, and the alert
through the hostname change detection with the notifier stand-in and no
resolver. The README describes the new behaviour.

Model: opus-5-5

TODO.md

@@ -23,6 +23,11 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
+- 2026-09-28: the inconsistency alert is sent once, on the check where two
+  nameservers start to disagree or where a nameserver that disagrees first
+  appears, instead of on every check while they disagree, and not again after
+  a restart. Every pair of nameservers is compared, not only neighbours in
+  sorted order of name (closes #158).
 - 2026-09-28: DNS names in record values (CNAME, MX, SRV and NS targets) are
   lower-cased, so nameservers that answer in different letter case no longer
   count as inconsistent or as a record change (closes #157).