watcher: change messages name only the record types that differ (closes #219)

A Record Change notification printed the nameserver's whole old and new
record sets in Go map syntax, and an Inconsistency notification the two
nameservers' whole sets, so a one-address change had to be found by eye
among kilobytes of unchanged TXT, CAA, MX and NS values. Both now list,
in sorted order of type, only the record types whose values differ: a
line naming the type, then each side's values separated by commas, or
none when that side has no records of that type. The dashboard's Recent
alerts shows the same text.

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: Record Change and Inconsistency notifications list only the record
+  types that differ, each with its values as plain text (closes #219).
 - 2026-10-02: a resolver test that reads one record type from a nameserver's
   answer asks again when that type is missing from it (closes #218).
 - 2026-10-02: a plain `docker build .` of a clone stamps its tag or short