Keep options in a fixed order, and say which block a diff hunk is in

Options of a block (server options, a view, a zone) are listed and
written grouped by topic, alphabetical within a group, so the same
option is in the same place everywhere. Top-level statements keep the
order they were added in.

named.conf diffs add the enclosing blocks to each hunk header, like
git's function context: @@ -50,6 +49,8 @@ view "internal" {.

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

internal/engine/diffcontext_test.go

@@ -0,0 +1,63 @@
+package engine
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestLabelHunks(t *testing.T) {
+	old := `// header
+options {
+	directory "/var/cache/bind";
+	forwarders { 1.1.1.1; 9.9.9.9; };
+	listen-on { any; };
+	recursion no;
+	version "a { b"; # a { in a string and comments: /* {
+	dnssec-validation auto;
+};
+
+view "internal" {
+	match-clients { 10.0.0.0/8; };
+	recursion yes;
+	allow-transfer { localhost; };
+	/* a { comment
+	   over lines */
+	zone "example.com" {
+		type primary;
+		file "example.com.db";
+		notify yes;
+	};
+	zone "other.example" { type primary; file "o.db"; };
+};
+`
+	new := strings.Replace(old, "\tforwarders { 1.1.1.1; 9.9.9.9; };\n", "", 1)
+	new = strings.Replace(new, "\tallow-transfer { localhost; };\n", "\tallow-transfer { localhost; };\n\tforward first;\n\tforwarders { 1.1.1.1; 9.9.9.9; };\n", 1)
+	new = strings.Replace(new, "\t\tnotify yes;\n", "\t\tnotify no;\n", 1)
+	d := fileDiff("/etc/bind/named.conf", []byte(old), true, []byte(new), false, false)
+	var headers []string
+	for _, l := range strings.Split(d, "\n") {
+		if strings.HasPrefix(l, "@@") {
+			headers = append(headers, l)
+		}
+	}
+	want := []string{
+		`@@ -1,7 +1,6 @@ options {`,
+		`@@ -12,12 +11,14 @@ view "internal" {`,
+	}
+	if strings.Join(headers, "\n") != strings.Join(want, "\n") {
+		t.Errorf("headers:\n%s\n\ndiff:\n%s", strings.Join(headers, "\n"), d)
+	}
+
+	// The first change of a hunk decides, and a change inside a zone
+	// names both blocks.
+	new2 := strings.Replace(old, "\t\tnotify yes;\n", "\t\tnotify no;\n", 1)
+	d = fileDiff("/etc/bind/named.conf", []byte(old), true, []byte(new2), false, false)
+	if !strings.Contains(d, ` @@ view "internal" { zone "example.com" {`+"\n") {
+		t.Errorf("zone change:\n%s", d)
+	}
+	// Zone files aren't named.conf syntax.
+	d = fileDiff("/var/lib/bind/example.com.db", []byte(old), true, []byte(new2), false, false)
+	if strings.Contains(d, "@@ view") {
+		t.Errorf("zone file labelled:\n%s", d)
+	}
+}