Server-sent events for token streams

Server-sent events are the streaming format that browsers already know how to read. The server sends a plain HTTP response with Content-Type: text/event-stream and writes events as short text blocks; the browser's EventSource object turns each block into a JavaScript event, reconnects if the connection drops, and tells the server where it left off. For a token stream, which only ever flows from server to client, that is everything you need and nothing you do not.

The format is so small that Relay does not need a library for it. It is JSON lines from the last lesson with a different frame: each event is one or more field: value lines followed by a blank line.