docs: HTTP push + SSE transport profile

[lodyai]

Adds http-push-sse-protocol.md describing an HTTP push + SSE transport profile that reuses the existing binary protocol.

Highlights:

• Join and client-originated update flows are expressed as HTTP request/response (JoinResponse*/Ack in the HTTP response body).
• Server→client traffic uses SSE events carrying base64url-encoded protocol frames.
• Notes on session binding, keepalive, ordering, and reconnect rejoin/backfill.

No wire format changes.