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>

internal/model/addr.go

@@ -6,6 +6,9 @@ import (
 	"strings"
 )
 
+// LinkLocal6 is IPv6's link-local prefix.
+var LinkLocal6 = netip.MustParsePrefix("fe80::/10")
+
 // Address is a parsed address entry: one address, a prefix or a range.
 type Address struct {
 	Prefix     netip.Prefix // a single address is a /32 or /128