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/report.go

@@ -0,0 +1,36 @@
+package backup
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Status is a single backup step's verdict. Unlike preflight, most backup
+// steps that fail are hard failures (Run aborts) rather than advisory - see
+// Run's doc comment - but Status still distinguishes "did the thing" from
+// "correctly skipped because it doesn't apply to this deployment".
+type Status string
+
+const (
+	StatusOK      Status = "ok"
+	StatusSkipped Status = "skipped"
+	StatusFail    Status = "fail"
+)
+
+type CheckResult struct {
+	Name   string
+	Status Status
+	Detail string
+}
+
+type Report struct {
+	Results []CheckResult
+}
+
+func (r Report) String() string {
+	var b strings.Builder
+	for _, res := range r.Results {
+		fmt.Fprintf(&b, "[%-7s] %-18s %s\n", strings.ToUpper(string(res.Status)), res.Name, res.Detail)
+	}
+	return b.String()
+}