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.go

@@ -0,0 +1,55 @@
+package preflight
+
+import (
+	"fmt"
+	"io/fs"
+	"path/filepath"
+	"syscall"
+)
+
+// DirSize walks dir and sums the size of every regular file in it. Used to
+// estimate how much free space a filesystem-level backup copy will need -
+// see ARCHITECTURE.md §4.2.
+func DirSize(dir string) (int64, error) {
+	var total int64
+	err := filepath.WalkDir(dir, func(_ string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.Type().IsRegular() {
+			info, err := d.Info()
+			if err != nil {
+				return err
+			}
+			total += info.Size()
+		}
+		return nil
+	})
+	if err != nil {
+		return 0, fmt.Errorf("preflight: measure size of %s: %w", dir, err)
+	}
+	return total, nil
+}
+
+// FreeBytes returns the free space available (to an unprivileged process)
+// on the filesystem containing path.
+func FreeBytes(path string) (uint64, error) {
+	var stat syscall.Statfs_t
+	if err := syscall.Statfs(path, &stat); err != nil {
+		return 0, fmt.Errorf("preflight: statfs %s: %w", path, err)
+	}
+	return stat.Bavail * uint64(stat.Bsize), nil
+}
+
+func humanBytes(n uint64) string {
+	const unit = 1024
+	if n < unit {
+		return fmt.Sprintf("%d B", n)
+	}
+	div, exp := uint64(unit), 0
+	for n/div >= unit {
+		div *= unit
+		exp++
+	}
+	return fmt.Sprintf("%.1f %ciB", float64(n)/float64(div), "KMGTPE"[exp])
+}