How do you make a private room in a place with no walls?

If you are on Bluesky, you already live in the open half of this story. Every post is a signboard outside your house that anyone can read. Feeds are little magazines that arrange those signboards into what you see. So how do you make something private in a town with no walls? Not a sealed envelope, which is encryption and a different problem, but a velvet rope: a room only members can enter, where the bouncer still has to recognize you to let you in. This is a tour, with metaphors instead of acronyms. Why you lock the room instead of locking every post. Why the room is imaginary, and your data never leaves your own server. How a short-lived wristband lets a brand-new app gather a whole forum on your behalf. And why the door is kept deliberately boring, so the interesting rules can live up in the apps. It includes a courier you can actually play with: hand it a wristband, watch it get bounced without one, flash an app ID badge at a picky club, and watch its wristband go stale mid-run. Built from Daniel Holmgren Permissioned Data Diary and the official draft proposal.