Refuse IPv4-mapped prefixes with an error that says so

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

CHANGELOG.md

@@ -162,6 +162,9 @@ All notable changes to Housecarl are documented here. The format follows
 
 ### Fixed
 
+- An IPv4-mapped prefix (`::ffff:0:0/96`) is refused with an error that
+  says so; it was refused as "has host bits set; the prefix is invalid
+  Prefix".
 - `hc forward show` shows the forward's hits and comment, and its
   hairpin as `no`, `yes` or `yes, from …` (it printed `no -`).
 - A history entry's ruleset diff showed the object it changed as moved: