Initial commit: stalwart-migrator design and scaffolding

In-place upgrade tool for Stalwart Mail Server (0.15.5 -> latest) with
checkpointed rollback and post-migration validation. Design stage; see
ARCHITECTURE.md.

internal/preflight/cluster_test.go

@@ -0,0 +1,34 @@
+package preflight
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLooksClustered(t *testing.T) {
+	cases := []struct {
+		name    string
+		content string
+		want    bool
+	}{
+		{"no-cluster", "[server]\nhostname = \"mail.example.com\"\n", false},
+		{"has-cluster-section", "[cluster]\nnode-id = 1\npeers = [\"10.0.0.2\"]\n", true},
+		{"cluster-mentioned-in-key", "[server]\ncluster-coordinator = \"redis://localhost\"\n", true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "config.toml")
+			if err := os.WriteFile(path, []byte(tc.content), 0o644); err != nil {
+				t.Fatal(err)
+			}
+			got, err := LooksClustered(path)
+			if err != nil {
+				t.Fatalf("LooksClustered: %v", err)
+			}
+			if got != tc.want {
+				t.Errorf("LooksClustered(%s) = %v, want %v", tc.name, got, tc.want)
+			}
+		})
+	}
+}