Stats heatmap: empty hours same height; cname_chain only for long chains

The "Busiest times" grid's hours without data used the class "empty",
which picked up the padding of empty messages. One CNAME to another no
longer warns; three or more get an info note showing the chain.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/validation.md

@@ -34,7 +34,7 @@ showing them.
 | `cname_multiple` | error | A name can have only one CNAME |
 | `cname_apex` | error | No CNAME at the zone apex (it needs SOA and NS) |
 | `cname_loop` | error | CNAMEs that point back to themselves |
-| `cname_chain` | warning | CNAME to a CNAME: works, but slows resolution |
+| `cname_chain` | info | a chain of three or more CNAMEs (one CNAME to another says nothing): works, but each is another lookup for some resolvers |
 | `cname_dangling` | warning | CNAME to a name that doesn't exist in the zone |
 | `dname_multiple`, `below_dname` | error | One DNAME per name, and nothing below a DNAME (RFC 6672) |
 | `apex_ns_missing` | error | A zone needs NS records at its apex |