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/diskspace_test.go

@@ -0,0 +1,55 @@
+package preflight
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestDirSize(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "a.bin"), make([]byte, 100), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	sub := filepath.Join(dir, "sub")
+	if err := os.Mkdir(sub, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(sub, "b.bin"), make([]byte, 250), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := DirSize(dir)
+	if err != nil {
+		t.Fatalf("DirSize: %v", err)
+	}
+	if got != 350 {
+		t.Errorf("DirSize = %d, want 350", got)
+	}
+}
+
+func TestFreeBytes(t *testing.T) {
+	got, err := FreeBytes(t.TempDir())
+	if err != nil {
+		t.Fatalf("FreeBytes: %v", err)
+	}
+	if got == 0 {
+		t.Error("FreeBytes = 0, want > 0 for a live filesystem")
+	}
+}
+
+func TestHumanBytes(t *testing.T) {
+	cases := []struct {
+		in   uint64
+		want string
+	}{
+		{500, "500 B"},
+		{1536, "1.5 KiB"},
+		{5 * 1024 * 1024, "5.0 MiB"},
+	}
+	for _, tc := range cases {
+		if got := humanBytes(tc.in); got != tc.want {
+			t.Errorf("humanBytes(%d) = %q, want %q", tc.in, got, tc.want)
+		}
+	}
+}