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/backup/binary_test.go

@@ -0,0 +1,63 @@
+package backup
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestPreserveBinary(t *testing.T) {
+	dir := t.TempDir()
+	binaryPath := filepath.Join(dir, "stalwart")
+	if err := os.WriteFile(binaryPath, []byte("fake binary"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	preserved, err := PreserveBinary(binaryPath, "0.15.5")
+	if err != nil {
+		t.Fatalf("PreserveBinary: %v", err)
+	}
+	wantPath := binaryPath + ".v0.15.5"
+	if preserved != wantPath {
+		t.Errorf("preserved = %s, want %s", preserved, wantPath)
+	}
+	if _, err := os.Stat(binaryPath); !os.IsNotExist(err) {
+		t.Error("original binary path should no longer exist after preservation")
+	}
+	if _, err := os.Stat(preserved); err != nil {
+		t.Errorf("preserved binary missing: %v", err)
+	}
+}
+
+func TestPreserveBinaryIsIdempotent(t *testing.T) {
+	dir := t.TempDir()
+	binaryPath := filepath.Join(dir, "stalwart")
+	if err := os.WriteFile(binaryPath, []byte("fake binary"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	first, err := PreserveBinary(binaryPath, "0.15.5")
+	if err != nil {
+		t.Fatalf("PreserveBinary #1: %v", err)
+	}
+
+	// Simulate a resumed run: the binary is already gone from binaryPath
+	// (moved on the prior attempt). Calling again must not error just
+	// because the source no longer exists.
+	second, err := PreserveBinary(binaryPath, "0.15.5")
+	if err != nil {
+		t.Fatalf("PreserveBinary #2 (resume): %v", err)
+	}
+	if first != second {
+		t.Errorf("preserved paths differ across calls: %s vs %s", first, second)
+	}
+}
+
+func TestPreserveBinaryRequiresSourceVersion(t *testing.T) {
+	dir := t.TempDir()
+	binaryPath := filepath.Join(dir, "stalwart")
+	os.WriteFile(binaryPath, []byte("x"), 0o755)
+	if _, err := PreserveBinary(binaryPath, ""); err == nil {
+		t.Fatal("PreserveBinary with an empty source version should error")
+	}
+}