Function createMemoryBackend

createMemoryBackend(options?: MemoryBackendOptions): MemoryBackend
A complete MessagingBackend that lives in memory.

Used by the shell's offline preview and by tests. Conversations are keyed direct:<pubkey> or group:<pubkey>,<pubkey> over the sorted correspondents. send() stores the message as sent, notifies subscribers and returns it; deliver() plays the other side. Unread counts are the number of received messages in a conversation. Cursors are numeric offsets.

send() rejects with invalid:no-recipients when neither a conversation nor a recipient is named, with invalid:unknown-conversation for an id the backend has never seen, and with unavailable:offline while available is false. Reads keep working offline so a preview can still show history.
Parameters
- options: MemoryBackendOptions = {}
  Owner key, availability and clock.
Returns MemoryBackend
A backend that also exposes deliver().
Example
```
const backend = createMemoryBackend({ owner: 'me', now: () => 1_700_000_000 });
await backend.send({ recipients: ['alice'], content: 'hi' });
backend.deliver({ senderPubkey: 'alice', content: 'hello' });
(await backend.conversations()).conversations[0];
// { id: 'direct:alice', kind: 'direct', participants: [{ pubkey: 'alice' }], unread: 1, updatedAt: 1700000000 }
```