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/report.go

@@ -0,0 +1,46 @@
+package preflight
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Status is a single check's verdict.
+type Status string
+
+const (
+	StatusOK   Status = "ok"
+	StatusWarn Status = "warn"
+	StatusFail Status = "fail"
+)
+
+// CheckResult is one named check's outcome.
+type CheckResult struct {
+	Name   string
+	Status Status
+	Detail string
+}
+
+// Report is the full set of preflight check outcomes.
+type Report struct {
+	Results []CheckResult
+}
+
+// Blocking reports whether any check failed hard enough that a run must
+// not proceed.
+func (r Report) Blocking() bool {
+	for _, res := range r.Results {
+		if res.Status == StatusFail {
+			return true
+		}
+	}
+	return false
+}
+
+func (r Report) String() string {
+	var b strings.Builder
+	for _, res := range r.Results {
+		fmt.Fprintf(&b, "[%-4s] %-20s %s\n", strings.ToUpper(string(res.Status)), res.Name, res.Detail)
+	}
+	return b.String()
+}