Retry events were previously fire-and-forget, so subscribers that
connected after a retry started only saw durable history plus
`status=running` and could not tell the stream was backing off.
Keep the current retry phase in `chatStreamState`, capture it atomically
with subscriber registration, replay it in the initial snapshot for
same-chat late joiners, and clear it when streaming resumes or ends so
reconnects get consistent retry state without duplicate delivery at the
subscription boundary.
Relates to CODAGT-139
Ethan
3a153ebb15