Build a production reference architecture for channel-based real-time messaging at hyperscale (think Slack workspaces or Discord guilds). The shape: tens of millions of long-lived TLS sockets, sub-second channel-broadcast delivery, server-side history that survives restarts, and a presence subsystem that does *not* melt when 50M users come online at once. Every component below is specified well enough that an SRE team would defend the choices in a 5-year incident retro — Slack and Discord have both already had those retros, public, and the canonical bakes their lessons in.