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/recovery/report.go

@@ -0,0 +1,31 @@
+package recovery
+
+import (
+	"fmt"
+	"strings"
+)
+
+type Status string
+
+const (
+	StatusOK   Status = "ok"
+	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, "[%-4s] %-16s %s\n", strings.ToUpper(string(res.Status)), res.Name, res.Detail)
+	}
+	return b.String()
+}