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/validate.go

@@ -189,7 +189,7 @@ func sortProblems(ps []Problem) {
 			return a.Zone < b.Zone
 		}
 		if a.Name != b.Name {
-			return model.CanonicalLess(a.Name, b.Name)
+			return model.NameLess(a.Name, b.Name)
 		}
 		return a.Message < b.Message
 	})