Refuse link-local addresses in the antispoof lists

fe80::/10 in the IPv6 antispoof list (or your prefixes) dropped the
ISP's router adverts and DHCPv6 replies in the early chain, before the
ICMPv6 and DHCPv6 client built-ins could accept them: the router lost
its IPv6 default route. Validation now refuses it
(antispoof_link_local), the settings page says so, and the importer
leaves it out with a note.

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

CHANGELOG.md

@@ -172,6 +172,13 @@ All notable changes to Housecarl are documented here. The format follows
 
 ### Fixed
 
+- Link-local addresses (`fe80::/10`) are refused in the IPv6 antispoof
+  list and in your prefixes (`antispoof_link_local`). Dropping them on
+  external zones dropped the ISP's router adverts and DHCPv6 replies
+  before the built-ins could accept them: the IPv6 default route expired
+  and, later, the delegated prefix would have. A configuration saved
+  with one keeps being applied, but the next change is refused until it
+  is taken out. `hc import nft` leaves them out, with a note.
 - 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".