queueSend abstract method
Queue a send for OFFLINE-first delivery (invariant 4): durably persist the
send INTENT (the same ZIP-321 URI) and return immediately — no network, no
signing, no money movement. It survives a process kill and proposes → signs
→ broadcasts on the next online sync (proposing at SEND time, not now, is
what stops a long-queued send from ever carrying a stale anchor). Returns
the opaque queued-send id; throws queuedSendsFull at the durable cap (a
real send is never silently dropped).
Implementation
Future<String> queueSend(String requestUri);