docs: cutover checklist and tsnats usage

Add docs/cutover.md (shadow :8001, embedded NATS :4223, REQUIRE_BEARER
host-token default, cutover/rollback; no JetStream for job-complete).
Expand README Usage with env table and pkg/tsnats snippet; add
examples/tsnats-bulk.

examples/tsnats-bulk/main.go

@@ -0,0 +1,50 @@
+// Command tsnats-bulk is a minimal digests-only stamp client against embedded NATS.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"time"
+
+	"git.georgelambert.org/marchon/verae-mcp-timestamping-server-go/pkg/tsnats"
+)
+
+func main() {
+	natsURL := flag.String("nats", envOr("NATS_URL", "nats://127.0.0.1:4223"), "NATS URL (embedded default :4223)")
+	digest := flag.String("digest", "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855", "SHA-256 hex")
+	scope := flag.String("scope", "local", "stamp scope: local|shared|global")
+	flag.Parse()
+
+	client, err := tsnats.Connect(*natsURL, tsnats.WithTimeout(5*time.Second))
+	if err != nil {
+		log.Fatalf("connect: %v", err)
+	}
+	defer client.Close()
+
+	ctx := context.Background()
+	sub, err := client.SubscribeCompleted(ctx, func(ev tsnats.CompletedEvent) {
+		fmt.Printf("completed jobId=%s receipt_id=%s hash=%s\n", ev.JobID, ev.ReceiptID, ev.Hash)
+	})
+	if err != nil {
+		log.Fatalf("subscribe completed: %v", err)
+	}
+	defer func() { _ = sub.Unsubscribe() }()
+
+	receipts, err := client.Stamp(ctx, *digest, tsnats.StampOptions{Scope: *scope})
+	if err != nil {
+		log.Fatalf("stamp: %v", err)
+	}
+	for _, r := range receipts {
+		fmt.Printf("receipt_id=%s hash=%s scope=%s\n", r.ReceiptID, r.Hash, r.Scope)
+	}
+}
+
+func envOr(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}