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/model/nameorder_test.go

@@ -0,0 +1,42 @@
+package model
+
+import (
+	"slices"
+	"testing"
+)
+
+func TestNameLess(t *testing.T) {
+	want := []string{
+		"10.0.10.in-addr.arpa.",
+		"1.10.0.10.in-addr.arpa.",
+		"02.10.0.10.in-addr.arpa.",
+		"2.10.0.10.in-addr.arpa.",
+		"10.10.0.10.in-addr.arpa.",
+		"101.10.0.10.in-addr.arpa.",
+		"x.101.10.0.10.in-addr.arpa.",
+		"254.10.0.10.in-addr.arpa.",
+		"a.10.0.10.in-addr.arpa.",
+	}
+	got := slices.Clone(want)
+	slices.Reverse(got)
+	slices.SortFunc(got, func(a, b string) int {
+		if NameLess(a, b) {
+			return -1
+		}
+		if NameLess(b, a) {
+			return 1
+		}
+		return 0
+	})
+	if !slices.Equal(got, want) {
+		t.Fatalf("got %v\nwant %v", got, want)
+	}
+
+	// Non-numeric labels keep canonical (byte) order.
+	if !NameLess("host10.example.", "host2.example.") {
+		t.Error("host10 should sort before host2")
+	}
+	if !NameLess("example.", "www.example.") {
+		t.Error("parent should sort before child")
+	}
+}