Sort numeric labels by value

Records, zones and findings now order all-digit labels numerically, so
reverse zones list 2 before 101. model.CanonicalLess is replaced by
model.NameLess; other labels keep byte order.

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

internal/validate/records.go

@@ -186,7 +186,7 @@ func (c *checker) checkZoneRecords(z *model.Zone) {
 	for n := range ix.types {
 		names = append(names, n)
 	}
-	sort.Slice(names, func(i, j int) bool { return model.CanonicalLess(names[i], names[j]) })
+	sort.Slice(names, func(i, j int) bool { return model.NameLess(names[i], names[j]) })
 
 	for _, name := range names {
 		types := ix.types[name]