watcher, config: no Record Change or Inconsistency for listed names (closes #255)

DNSWATCHER_SKIP_RECORD_NOTIFICATIONS takes a comma-separated list of
names from DNSWATCHER_TARGETS, read as the targets are (letter case,
trailing dot, repeats). For a listed name no Record Change and no
Inconsistency notification is sent; its records are still checked and
saved, and its other notifications are sent. A listed name that is not
a target stops startup with an error naming it.

The two detections return early for a listed name. Apex domains are
covered too: their own records go through the same detection.

Live DNS cannot be made to disagree on purpose, so the Inconsistency
test feeds records to the change detection directly, as the existing
inconsistency tests do; the live test covers Record Change.

Model: opus-5-5

TODO.md

@@ -19,6 +19,9 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-05: a name listed in `DNSWATCHER_SKIP_RECORD_NOTIFICATIONS` gets no
+  Record Change or Inconsistency notification; a name listed there that is not
+  in `DNSWATCHER_TARGETS` stops startup (closes #255).
 - 2026-10-02: a nameserver whose query for one record type failed while the
   others answered with no records is `ok`, not `nodata` (closes #253).
 - 2026-10-02: a domain that does not exist is shown so, with no nameservers; no