Envelopes and message types

The TypeScript types in @cachepuppy/core describe the on-wire JSON:

• v: protocol version, currently 1.
• type: one of subscribe, unsubscribe, publish, set_state, get_state, close_topic, system, …
• id: string identifier allocated per message.
• topic: logical topic name for topic-scoped messages.
• event: application event name for publishes and some system messages.
• payload: arbitrary JSON-compatible payload.
• ts: millisecond timestamp.
• meta: optional map — Phoenix adds fields such as clientId for publishes.