resolver: store a name's CNAME once per nameserver (closes #220)

For a name with a CNAME, a nameserver answers a query of any type with
that CNAME, and the records of every answer were added, so the CNAME
was stored once for each of the eight record types asked for.
collectAnswerRecords now adds each value once per record type.

A state file saved before this holds the repeated values. Load keeps
each record value once, so the first check after upgrading sees no
record change and notifies nothing for them.

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: a name's CNAME is stored once per nameserver, not once per record
+  type asked for; a state file with repeats loads each value once (closes #220).
 - 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