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.go

@@ -0,0 +1,31 @@
+package backup
+
+import (
+	"fmt"
+	"os"
+)
+
+// PreserveBinary moves the currently-installed binary aside to
+// "<binaryPath>.v<sourceVersion>" so rollback can restart the exact old
+// binary without re-downloading anything, and cutover can install the new
+// one at the original path. It never deletes the old binary, and it's
+// idempotent: if a prior attempt at this run already preserved it, calling
+// this again just returns the existing preserved path rather than erroring
+// on a missing source file.
+func PreserveBinary(binaryPath, sourceVersion string) (preservedPath string, err error) {
+	if sourceVersion == "" {
+		return "", fmt.Errorf("backup: cannot preserve %s without a source version to suffix it with", binaryPath)
+	}
+	preservedPath = binaryPath + ".v" + sourceVersion
+
+	if _, statErr := os.Stat(preservedPath); statErr == nil {
+		return preservedPath, nil
+	} else if !os.IsNotExist(statErr) {
+		return "", fmt.Errorf("backup: stat %s: %w", preservedPath, statErr)
+	}
+
+	if err := os.Rename(binaryPath, preservedPath); err != nil {
+		return "", fmt.Errorf("backup: preserve %s as %s: %w", binaryPath, preservedPath, err)
+	}
+	return preservedPath, nil
+}